Communicable Diseases

** Explanation of Terms **

Technical meaning of some terms used in CCDM (not binding definitions)

  1. Carrier- A person or animal that harbors a specific infectious agent without discernible clinical disease, and which serves as a potential source of infection. The carrier state may exist in an individual with an infection that is unapparent throughout its course (such an individual is commonly known as healthy or asymptomatic carrier), or during the incubation period, convalescence and post-convalescence of a person with a clinically recognizable disease (commonly known as an incubatory or convalescent carrier). Under either circumstance the carrier state may be of short or long duration (temporary or transient carrier, or chronic carrier).

  2. Case-fatality rate- (synonyms: fatality rate, fatality percentage, case-fatality ratio) Usually expressed as the proportion of persons diagnosed as having a specified disease who die within a given period as a result of acquiring that disease. In communicable disease epidemiology, this term is most frequently applied to a specific outbreak of acute disease in which all patients have been followed for a period of time sufficient to include all deaths attributable to the given disease. The case-fatality rate – where the numerator is “deaths from a given disease in a given period” and the denominator is “number of diagnosed cases of the disease during that period” – must be differentiated from the disease-specific mortality rate, where the denominator is “total population”.

  3. Chemoprophylaxis- The administration of a chemical, including antibiotics, to prevent the development of an infection or the progression of an infection to active manifest disease, or to eliminate the carriage of a specific infectious agent in order to prevent transmission and disease in others. Chemotherapy refers to use of a chemical to treat a clinically manifest disease or to limit its further progress.

  4. Cleaning- The removal by scrubbing and washing, as with water, soap, antiseptic or suitable detergent or by vacuum cleaning, of infectious agents and of organic matter from surfaces on which and in which infectious agents may find favorable conditions for surviving or multiplying.
    • Terminal cleaning is the cleaning after the patient has been removed by death or transfer, or has ceased to be a source of infection, or after hospital isolation or other practices have been discontinued. See also Terminal disinfection.


  5. Communicable disease- (synonym: infectious disease) An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal or inanimate source to a susceptible host; either directly or indirectly through an intermediate plant or animal host, through a vector, or through contact with the inanimate environment.

  6. Contact- In the context of communicable disease, a person or animal that has been in association with an infected person or animal or a contaminated environment, and so has had an opportunity to acquire the infection.

  7. Contamination- The presence of an infectious agent on a body surface, in or on clothes, bedding, toys, surgical instruments or dressings, or in other inanimate articles or substances including water, milk and food. Contamination of a body surface does not imply a carrier state. Pollution is distinct from contamination and implies the presence of offensive, but not necessarily infectious, matter in the environment.

  8. Disinfection- Killing of infectious agents outside the body by direct exposure to chemical or physical agents. High-level disinfection may kill all microorganisms with the exception of high numbers of bacterial spores; extended exposure is required to ensure killing of most bacterial spores. High-level disinfection is achieved, after thorough detergent cleaning, through exposure to specific concentrations of certain disinfectants (e.g., 2% glutaralde-hyde, 6% stabilized hydrogen peroxide and up to 1% peracetic acid) for at least 20 minutes. Intermediate-level disinfection does not kill spores; it can be achieved through pasteurization (75°C [167°F] for 30 minutes) or appropriate treatment with approved disinfectants.
    • Concurrent disinfection is the application of disinfective measures as soon as possible after the discharge of infectious material from the body of an infected person, or after the soiling of articles with such infectious discharges; all personal contact with such discharges or articles should be minimized prior to concurrent disinfection.

    • Terminal disinfection is the application of disinfective measures after the patient has been removed by death or transfer, or has ceased to be a source of infection, or after hospital isolation or other practices have been discontinued. Terminal disinfection is rarely practiced; terminal cleaning generally suffices (see Cleaning), along with airing and sunning of rooms, furniture and bedding. Steam sterilization or incineration of bedding and other items is sometimes recommended after a disease such as Lassa fever or another highly infectious disease.

    • Sterilization involves destruction of all forms of microbial life by physical heat, irradiation, gas or chemical treatment.


  9. Disinfestation- Any physical or chemical process serving to destroy or remove undesired small animal forms, particularly arthropods or rodents, present upon the person or clothing of an individual, or in the environment (see Insecticide and Rodenticide). Disinfestation includes delousing for infestation with Pediculus humanus, the human body louse. Synonyms include the terms disinsection and disinsectization when only insects are involved.

  10. Endemic- A term denoting the habitual presence of a disease or infectious agent within a given geographic area or a population group; may also refer to the usual prevalence of a given disease within such an area. Hyperendemic expresses a habitual presence at all ages at a high level of incidence, and holoendemic (a term applied mainly to malaria) expresses a high level of prevalence with high spleen rates in children and lower rates in adults. (See also Zoonosis.)

  11. Epidemic- The occurrence, in a defined community or region, of cases of an illness (or an outbreak) with a frequency clearly in excess of normal expectancy. The number of cases indicating the presence of an epidemic varies according to the infectious agent, size and type of population exposed, previous experience of or lack of exposure to the disease, and time and place of occurrence; epidemicity is thus relative to usual frequency of the disease in the same area, among the specified population, at the same season of the year. A single case of a communicable disease long absent from a population or the first invasion by a disease not previously recognized in that area requires immediate reporting and full field epidemiological investigation; 2 cases of such a disease associated in time and place are sufficient evidence of transmission to be considered an epidemic (see Report of a Disease and Zoonosis).

  12. Food irradiation- A technique that provides a specific dose of ionizing radiation from a source such as a radioisotope (e.g., cobalt 60), or from machines that produce accelerated electron beams or X-rays. Doses for irradiation of food and material are: low – 1 or less kiloGrays (kGy), used for disinfestation of insects from fruit, spices and grain and for parasite disinfection in fish and meat; medium – 1–10 kGy (commonly 1– 4 kGy), used for pasteurization and the destruction of bacteria and fungi; and high – 10 –50 kGy, used for sterilization of food as well as medical supplies (including IV fluids, implants, syringes, needles, thread, clips and gowns).

  13. Fumigation- A process by which the killing of animal forms, especially arthropods and rodents, is accomplished by the use of gaseous agents (see Insecticide and Rodenticide).

  14. Health education- (synonyms: patient education, education for health, education of the public, public health education) The process by which individuals and groups of people learn to behave in a manner conducive to the promotion, maintenance or restoration of health. Education for health begins with people as they are, with whatever interests they may have in improving their living conditions. Its aim is to develop their sense of their own responsibility for health conditions, as individuals and as members of families and communities. In communicable disease control, health education commonly includes an appraisal of what is known by a population about a disease, an assessment of habits and attitudes of the people as they relate to spread and frequency of the disease, and the presentation of specific means to remedy observed deficiencies.

  15. Herd immunity- The immunity of a group or community. The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group.

  16. Host- A person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural (as opposed to experimental) conditions. Some protozoa and helminths pass successive stages in alternate hosts of different species. Hosts in which a parasite attains maturity or passes its sexual stage are primary or definitive hosts; those in which a parasite is in a larval or asexual state are secondary or intermediate hosts. A transport host is a carrier in which the organism remains alive but does not undergo development.

  17. Immune individual- A person or animal that has specific protective antibodies and/or cellular immunity as a result of previous infection or immunization, or is so conditioned by such previous specific experience as to respond in a way that prevents the development of infection and/or clinical illness following re-exposure to the specific infectious agent. Immunity is relative: a level of protection that could be adequate under ordinary conditions may be overwhelmed by an excessive dose of the infectious agent or by exposure through an unusual portal of entry; protection may also be impaired by immunosuppressive drug therapy, concurrent disease or the ageing process.

  18. Immunity- A status usually associated with the presence of antibodies or cells having a specific action on the microorganism concerned with a particular infectious disease, or on its toxin. Effective immunity includes both cellular immunity, conferred by T-lymphocyte sensitization, and humoral immunity, based on B-lymphocyte response. Passive immunity is attained either naturally through trans-placental transfer from the mother, or artificially by inoculation of specific protective antibodies (from immunized animals, or convalescent hyperimmune serum or immune serum globulin [human]). Passive immunity is of short duration (days to months). Active humoral immunity, which usually lasts for years, is attained either naturally through infection with or without clinical manifestations, or artificially through inoculation of the agent itself in killed, modified or variant form, or of fractions or products of the agent.

  19. Inapparent infection- (synonyms: asymptomatic, sub-clinical, occult, or unapparent infection) See Unapparent infection.

  20. Incidence- The number of instances of illness commencing, or of persons falling ill, during a given period in a specified population. The incidence rate is the ratio of new cases of a specified disease diagnosed or reported during a defined period of time to the number of persons at risk in a stated population in which the cases occurred during the same period of time (if the period is one year, the rate is the annual incidence rate). This rate is usually expressed as cases per 1 000 or 100 000 per annum, for the whole population, or specifically for any population characteristic or subdivision such as age or ethnic group (see Prevalence rate). Attack rate, or case rate, is a proportion measuring cumulative incidence for a particular group, over limited periods and under special circumstances, as in an epidemic; it is usually expressed as a percentage (cases per 100 in the group). The numerator can be determined through the identification of clinical cases or through seroepidemiology. The secondary attack rate is the ratio of the number of cases among contacts occurring within the accepted incubation period following exposure to a primary case to the total number of exposed contacts; the denominator may be restricted to the numbers of susceptible contacts when this can be determined. The infection rate is a proportion that expresses the incidence of all identified infections, manifest or unapparent (the latter identified by seroepidemiology).

  21. Incubation period- The time interval between initial contact with an infectious agent and the first appearance of symptoms associated with the infection. In a vector, it is the time between entrance of an organism into the vector and the time when that vector can transmit the infection (extrinsic incubation period). The period between the time of exposure to an infectious agent and the time when the agent can be detected in blood or stool is called the prepatent period.

  22. Infected individual- A person or animal that harbors an infectious agent and who has either manifest disease or unapparent infection (see Carrier). An infectious person or animal is one from whom the infectious agent can be naturally acquired.

  23. Infection- The entry and development or multiplication of an infectious agent in the body of persons or animals. Infection is not synonymous with infectious disease; the result may be unapparent (see Unapparent infection) or manifest (see Infectious disease). The presence of living infectious agents on exterior surfaces of the body, or on articles of apparel or soiled articles, is not infection, but represents contamination of such surfaces and articles (see Infestation and Contamination).

  24. Infectious agent- An organism (virus, rickettsia, bacteria, fungus, protozoan or helminth) that is capable of producing infection or infectious disease. Infectivity expresses the ability of the infectious agent to enter, survive and multiply in the host. Infectiousness indicates the relative ease with which an infectious agent is transmitted to other hosts.

  25. Infectious disease- A clinically manifest disease of humans or animals resulting from an infection (see Infection).

  26. Infestation- For persons or animals, the lodgment, development and reproduction of arthropods on the surface of the body or in the clothing. Infested articles or premises are those that harbor or give shelter to animal forms, especially arthropods and rodents.

  27. Insecticide- Any chemical substance used for the destruction of insects; can be applied as powder, liquid, atomized liquid, aerosol or “paint” spray; an insecticide may or may not have residual action. The term larvicide is generally used to designate insecticides applied specifically for the destruction of immature stages of arthropods; adulticide or imagocide, to those destroying mature or adult forms. The term insecticide is used broadly to encompass substances for the destruction of all arthropods; acaricide is more properly used for agents against ticks and mites. Specific terms such as lousicide and miticide are sometimes used.

  28. Isolation- As applied to patients, isolation represents separation, for a period at least equal to the period of communicability, of infected persons or animals from others, in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible to infection or who may spread the agent to others.
    • Universal precautions should be used consistently for all patients (in hospital settings as well as outpatient settings) regardless of their blood-borne infection status. This practice is based on the possibility that blood and certain body fluids (any body secretion that is obviously bloody, semen, vaginal secretions, tissue, CSF, and synovial, pleural, peritoneal, pericardial and amniotic fluids) of all patients are potentially infectious for agents such as HIV, HBV and other blood-borne pathogens. Universal precautions are intended to prevent parenteral, mucous membrane and non-intact skin exposures of health care workers to blood-borne pathogens. Protective barriers include gloves, gowns, masks and protective eyewear or face shields. A private room is indicated if patient hygiene is poor. Local and state authorities control waste management. Two basic requirements are common for the care of all potentially infectious cases:
      1. Hands must be washed after contact with the patient or potentially contaminated articles and before taking care of another patient

      2. Articles contaminated with infectious material must be appropriately discarded or bagged and labeled before being sent for decontamination and reprocessing.

      Recommendations made for isolation of cases in section 9B2 of each disease may allude to the methods that have been recommended as category-specific isolation precautions, based on the mode of transmission of the specific disease, in addition to universal precautions. These categories are as follows:
      • Strict isolation: To prevent transmission of highly contagious or virulent infections that may be spread by both air and contact. The specifications, in addition to those above, include a private room and the use of masks, gowns and gloves for all persons entering the room. Special ventilation requirements with the room at negative pressure to surrounding areas are desirable.

      • Contact isolation: For less highly transmissible or less serious infections, or for diseases or conditions that are spread primarily by close or direct contact. In addition to the 2 basic requirements, a private room is indicated, but patients infected with the same pathogen may share a room. Masks are indicated for those who come close to the patient, gowns if soiling is likely, and gloves for touching infectious material.

      • Respiratory isolation: To prevent transmission of infectious diseases over short distances through the air, a private room is indicated, but patients infected with the same organism may share a room. In addition to the basic requirements, masks are indicated for those who come in close contact with the patient; gowns and gloves are not indicated.

      • Tuberculosis isolation (AFB isolation): For patients with pulmonary tuberculosis who have a positive sputum smear or a chest X-ray that strongly suggests active tuberculosis. Specifications include use of a private room with special ventilation and closed doors. In addition to the basic requirements, those entering the room must use respirator-type masks. The use of gowns will prevent gross contamination of clothing. Gloves are not indicated.

      • Enteric precautions: For infections transmitted by direct or indirect contact with feces. In addition to the basic requirements, specifications include use of a private room if patient hygiene is poor. Masks are not indicated; gowns should be used if soiling is likely and gloves should be used when touching contaminated materials.

      • Drainage/secretion precautions: To prevent infections transmitted by direct or indirect contact with purulent material or drainage from an infected body site. A private room and masking are not indicated. In addition to the basic requirements, gowns should be used if soiling is likely and gloves used when touching contaminated materials.



  29. Molluskicide- A chemical substance used for the destruction of snails and other mollusks.

  30. Mortality rate- (synonym: death rate) A rate calculated in the same way as an incidence rate, by dividing the number of deaths occurring in the population during the stated period of time, usually a year, by the number of persons at risk of dying during the period or by the mid-period population. A total or crude mortality rate refers to deaths from all causes and is usually expressed as deaths per 1 000. A disease-specific mortality rate refers to deaths due to a single disease and is often reported for a denominator of 100 000 persons. Age, ethnicity or other characteristics may define the population base. The mortality rate must not be confused with the case-fatality rate.

  31. Nosocomial infection- (synonym: hospital-acquired infection) An infection occurring in a patient in a hospital or other health care facility in whom the infection was not present or incubating at the time of admission; or the residual of an infection acquired during a previous admission. Includes infections acquired in the hospital but appearing after discharge, and also such infections among the staff of the facility.

  32. Pathogenicity- The property of an infectious agent that determines the extent to which overt disease is produced in an infected population, or the power of an organism to produce disease. Measured by the ratio of the number of persons developing clinical illness to the number of persons exposed to infection.

  33. Period of communicability/Communicable period- The time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to humans, or from an infected person to animals, including arthropods. In diseases (e.g., diphtheria and streptococcal infection) where mucous membranes are involved from the initial entry of the infectious agent, the period of communicability starts at the date of first exposure to a source of infection and lasts until the infecting microorganism is no longer disseminated from the mucous membranes, i.e. from the period before the prodromata until the termination of a carrier state, if the latter develops. Some diseases (e.g., hepatitis A, measles) are more easily communicable during the incubation period than during the actual illness. In diseases such as tuberculosis, leprosy, syphilis, gonorrhea and some of the salmonel-loses, the communicable state may persist—sometimes intermittently—over a long period, with discharge of infectious agents from the surface of the skin or through the body orifices. For diseases transmitted by arthropods, such as malaria and yellow fever, the periods of communicability (or infectivity) are those during which the infectious agent occurs in the blood or other tissues of the infected person in sufficient numbers to permit infection of the vector. For the arthropod vector, a period of communicability (transmissibility) is also to be noted, during which the agent is present in the tissues of the arthropod in such form and locus as to be transmissible (infective state).

  34. Personal hygiene- In the field of infectious disease control, those protective measures, primarily within the responsibility of the individual, that promote health and limit the spread of infectious diseases, chiefly those transmitted by direct contact. Such measures encompass:
    • Washing hands in soap and water immediately after evacuating bowel or bladder and always before handling food or eating

    • Keeping hands and unclean articles, or articles that have been used for toilet purposes by others, away from the mouth, nose, eyes, ears, genitalia and wounds

    • Avoiding the use of common or unclean eating utensils, drinking cups, towels, handkerchiefs, combs, hairbrushes and pipes

    • Avoiding exposure of other persons to droplets from the nose and mouth expelled when coughing, sneezing, laughing or talking

    • Washing hands thoroughly after handling a patient or a patient's belongings, and keeping the body clean by frequent soap and water washing.


  35. Prevalence- The total number of instances of illness or of persons ill in a specified population at a particular time (point prevalence), or during a stated period of time (period prevalence), without distinction between old and new cases. A prevalence rate (not to be confused with prevalence) is the ratio of prevalence to the population at risk of having the disease or condition at the stated point in time or midway through the period considered; it is usually expressed per 1 000, per 10 000 or per 100 000 population.

  36. Quarantine- Restriction of activities for well persons or animals who have been exposed (or are considered to be at high risk of exposure) to a case of communicable disease during its period of communicability (i.e. contacts), to prevent disease transmission during the incubation period if infection should occur. The two main types of quarantine are:
    • Absolute or complete quarantine: The limitation of freedom of movement of those exposed to a communicable disease for a period of time not longer than the longest usual incubation period of that disease, in such a manner as to prevent effective contact with those not so exposed (see Isolation).

    • Modified quarantine: A selective, partial limitation of freedom of movement of contacts, commonly on the basis of known or presumed differences in susceptibility and related to the assessed risk of disease transmission. It may be designed to accommodate particular situations. Examples are exclusion of children from school, exemption of immune persons from provisions applicable to susceptible persons, or restriction of military populations to post or to quarters. Modified quarantine includes: personal surveillance, the practice of close medical or other supervision of contacts to permit prompt recognition of infection or illness but without restricting their movements; and segregation, the separation of some part of a group of persons or domestic animals from the others for special consideration, control or observation; removal of susceptible children to homes of immune persons; or establishment of a sanitary boundary to protect uninfected from infected portions of a population.


  37. Repellent- A chemical applied to the skin or clothing or other places to discourage arthropods from alighting on and biting a person, or to discourage other agents, such as helminth larvae, from penetrating the skin.

  38. Report of a disease- An official report notifying an appropriate authority of the occurrence of a specified communicable or other disease in humans or in animals. Diseases in humans are reported to the local health authority; those in animals, to the livestock, sanitary, veterinary or agriculture authority. Some few diseases in animals, also transmissible to humans, are reportable to both authorities. Each health jurisdiction declares a list of reportable diseases appropriate to its particular needs (see the chapter). Reports should also list suspected cases of diseases of particular public health importance, ordinarily those requiring epidemiological investigation or initiation of special control measures. When a person is infected in one health jurisdiction and the case is reported from another, the health authority receiving the report should notify the jurisdiction where infection presumably occurred, especially when the disease requires examination of contacts for infection, or if food, water or other common vehicles of infection may be involved. In addition to routine reports of cases of specified diseases, special notification is required of most epidemics or outbreaks of disease, including diseases not listed as reportable (see Epidemic). Special reporting requirements are specified in chapters on the International Health Regulations (2005) and Reporting of communicable diseases.
    • Zero reporting (synonym: null reporting) consists of the explicit reporting of “zero cases” when no cases have been detected by the reporting unit. This is a way of checking that the relevant data have not been forgotten or lost.


  39. Reservoir- (of infectious agents) Any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, or where it reproduces itself in such manner that it can be transmitted to a susceptible host.

  40. Rodenticide- A substance used for the destruction of rodents, generally but not always through ingestion (see also Fumigation.)

  41. Source of infection- The person, animal, object or substance from which an infectious agent passes to a host. Source of infection should be clearly distinguished from source of contamination, such as overflow of a septic tank contaminating a water supply (see Reservoir).

  42. Surveillance of disease- In communicable disease control, surveillance consists of the process of systematic collection, orderly consolidation and analysis and evaluation of pertinent data with prompt dissemination of the results to those who need to know them, and particularly those who are in a position to take action. It includes the systematic collection and evaluation of:
    1. Morbidity and mortality reports

    2. Special reports of field investigations of epidemics and of individual cases

    3. Isolation and identification of infectious agents by laboratories

    4. Data concerning the availability, use and untoward effects of vaccines and toxoids, immune globulins, insecticides and other substances used in control

    5. Information regarding immunity levels in segments of the population

    6. Other relevant epidemiological data.

    A report summarizing the above data should be prepared and distributed to all cooperating persons and others with a need to know the results of the surveillance activities. The procedure applies to all jurisdictional levels of public health, from local to international.
    • Serological surveillance identifies patterns of current and past infection using serological tests for antibody detection.


  43. Susceptible- A person or animal not possessing sufficient resistance to a particular infectious agent to prevent contracting infection or disease when exposed to that agent.

  44. Suspect- In the context of infectious disease control, illness in a person whose history and symptoms suggest that he or she may have, or be developing, a communicable disease.

  45. Terminal cleaning- See Cleaning.

  46. Terminal disinfection- See Disinfection.

  47. Transmission of infectious agents- Any mechanism by which an infectious agent is spread from a source or reservoir to a person. These mechanisms are as follows:
    • Direct transmission: Direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place. This may be by direct contact such as touching, biting, kissing or sexual intercourse, or through direct projections (droplet spread) of droplet spray onto the conjunctiva or onto the mucous membranes of the eye, nose or mouth during sneezing, coughing, spitting, singing or talking (risk of transmission in this manner is usually limited to a distance of about l meter or less from the source of infection). Direct transmission may also occur through direct exposure of susceptible tissue to an agent in soil, through the bite of a rabid animal, or trans-placentally.

    • Indirect transmission:
      • -Vehicle-borne—Contaminated inanimate materials or objects (fomites) such as toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, surgical instruments or dressings; water, food, milk, and biological products including blood, serum, plasma, tissues or organs; or any substance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry. The agent may or may not have multiplied or developed in or on the vehicle before being transmitted.

      • -Vector-borne
        1. Mechanical: Includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis, or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism.

        2. Biological: Propagation (multiplication), cyclic development, or a combination of these (cyclopropagative) is required before the arthropod can transmit the infective form of the agent to humans. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (transovarian transmission); trans-stadial transmission indicates its passage from one stage of the life cycle to another, as from nymph to adult. Transmission may be by injection of salivary gland fluid during biting, or by regurgitation or deposition on the skin of feces or other material capable of penetrating through the bite wound or through an area of trauma, often created by scratching or rubbing. This transmission is by an infected nonvertebrate host and not simple mechanical carriage by a vector as a vehicle. An arthropod in either role is termed a vector.
          • Airborne transmission: The dissemination of microbial aerosols to a suitable portal of entry, usually the respiratory tract. Microbial aerosols are suspensions of particles in the air consisting partially or wholly of microorganisms. They may remain suspended in the air for long periods of time, some retaining and others losing infectivity or virulence. Particles in the 1-to 5-micrometer range are easily drawn into the alveoli of the lungs and may be retained there. Not considered as airborne are droplets and other large particles that promptly settle out (see Direct transmission).



      • Droplet nuclei—Usually the small residues that result from evaporation of fluid from droplets emitted by an infected host (see above). They may also be created purposely by a variety of atomizing devices, or accidentally as in microbiology laboratories, abattoirs, rendering plants or autopsy rooms. They usually remain suspended in the air for long periods.

      • Dust—The small particles of widely varying size that may arise from soil (e.g., fungus spores), clothes, bedding or contaminated floors.



  48. Unapparent infection- (synonyms: asymptomatic, inapparent, sub-clinical, or occult infection) The presence of infection in a host without recognizable clinical signs or symptoms. Unapparent infections are identifiable only through laboratory means such as a blood test, or through the development of positive reactivity to specific skin tests.

  49. Universal precautions- See Isolation.

  50. Virulence- The ability of an infectious agent to invade and damage tissues of the host; the degree of pathogenicity of an infectious agent, often indicated by case-fatality rates.

  51. Zoonosis- An infection or infectious agent transmissible under natural conditions from vertebrate animals to humans. May be enzootic or epizootic (see Endemic and Epidemic).

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