Severe acute viral illnesses, usually with sudden onset of fever, malaise, myalgia and headache, followed by pharyngitis, vomiting, diarrhea and maculopapular rash. In severe and fatal forms, the hemorrhagic diathesis is often accompanied by hepatic damage, renal failure, CNS involvement and terminal shock with multi-organ dysfunction. Laboratory findings usually show lymphopenia, severe thrombocytopenia and transaminase elevation (AST greater than ALT), sometimes with hyperamylasemia, elevated creatinine and blood urea nitrogen levels during the final renal failure phase. Case-fatality rates for Ebola infections in well-studied outbreaks in Africa have ranged from 50% to nearly 90%; 25%–80% of reported cases of Marburg virus infection have been fatal.

Diagnosis is usually through a combination of assays detecting antigen or RNA and antibody IgM or IgG. RT-PCR or ELISA antigen detection can be used on blood, serum or organ homogenates (the presence of IgM antibody suggests recent infection). Virus isolation attempts in cell culture or suckling mice must be undertaken in a BSL-4 laboratory. ELISA is used for specific IgM and IgG antibody detection in serum (the presence of IgM antibody suggesting recent infection). Virus may sometimes be visualized in liver, spleen, skin and other tissue sections by EM. Post-mortem diagnosis through immunohistochemical examination of formalin-fixed skin biopsy or autopsy specimens is possible. IFA tests for antibodies have often been misleading, particularly in serological surveys for past infection. Laboratory studies represent an extreme biohazard, and should be carried out only where protection against infection of the staff and community is available (BSL-4 containment).