Let the incubation period do the work

Tropical medicine looks like an impossible list of diseases until you notice that time does most of the sorting. Before you reach for a differential, pin down two dates: when exposure could have begun and when symptoms actually started. The interval between them quietly deletes half the list.

A rough working sieve:

• Under about 10 days: dengue, chikungunya, rickettsial infection, other arboviral illness — and the everyday causes of fever that travel does not exempt you from.
• Around 7–21 days: malaria (falciparum most often within a month), enteric fever, leptospirosis; acute schistosomiasis usually comes later, at 2–12 weeks.
• Beyond a month: vivax and ovale malaria with their liver stages, tuberculosis, visceral leishmaniasis, amoebic liver abscess, chronic schistosomiasis. Viral haemorrhagic fevers, by contrast, are effectively excluded beyond 21 days from exposure — which is exactly why the exposure date matters so much.

The history that feeds the sieve is not “did you travel?” but the specifics: rural or urban, freshwater contact, mosquito bites by day or by night, unpasteurised milk, sexual contact, animal contact, healthcare contact, and what prophylaxis and vaccines were actually taken.

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