Similarly infections with short incubation periods may have been acquired locally after return from abroad. The period of time after leaving the tropical and developing countries Knowledge of the time interval between the patient leaving an endemic area and the onset of illness, and the incubation periods of relevant infections, may allow exclusion of numerous possible diagnoses without extensive investigations. For example, an incubation period of three weeks or more excludes many arboviruses such as dengue, yellow fever, and viral haemorrhagic fevers such as Lassa fever. Most infections become manifest within three weeks of exposure and the majority of acute life-threatening infections will appear within 3 to 6 months. Infections presenting more than 6 months after exposure tend to have a subacute presentation and clinical course.
Some organisms have the potential to persist indefinitely in the human host.
Fungi Coccidioides immitis Histoplasma capsulatum 2.4. Where the traveller has been is critical to the differential diagnosis, eg. Japanese B encephalitis is confined to Asia, and Lassa fever is restricted to West Africa. Types of exposure Knowledge of the patient's activities is highly relevant, including level of hygiene, occupation, types of food eaten, insect exposure, skin-piercing procedures and sex. Health workers are at risk of nosocomial transmission of Lassa fever and other viral haemorrhagic fevers.