In natural disasters, most deaths occur in the immediate aftermath of the disaster and are primarily due to trauma and drowning. There may be population displacement after the disaster, and breakdowns in essential services such as shelter, water, sanitation and health care provision. The communicable disease risk depends on the extent of the displacement and the baseline health status of the affected population. Specific risks may be influenced by the type of disaster—for example, flooding often leads to the increased risk of water-related diseases.

In conflict and post-conflict situations, the magnitude of population displacement is often greater and more prolonged. These populations often have poor health status in advance of the emergency, and children are at particular risk. Highest excess mortality and morbidity most commonly occur during the first few weeks after acute conflict. Death rates of over 60 times the baseline have been recorded in refugee populations, with over three-quarters of these deaths caused by communicable diseases.

The factors determining the communicable disease risk in both natural disaster and conflict situations are as follows:

1. Population displacement.
   
   
2. Baseline health status including prevalence of malnutrition.
   
   
3. Lack of access to shelter, food, water and sanitation.
   
   
4. Overcrowding in temporary settlements or camps.
   
   
5. Lack of access to basic health services—e.g. primary health care and immunization.