Of the four species of the malaria parasite (spread by mosquitos), Plasmodium falciparum is the most common and also most fatal. It begins as a minor fever like a virus, but on the 3rd day can become overwhelming and attack the brain. If a patient has just returned from a tropical country within the last 40 days, unless they’d only stayed in a major city, we assume any fever to be malaria until proved otherwise. If they took preventive medicine (prophylaxis) as they certainly should have, it can still be malaria, but wouldn’t kill as quickly.
Those at highest risk for death are healthy tourists without immunity from old infections; also immigrants who’d returned to visit home, but had been away long enough [a few years] for their immunity to fade. We order blood tests for malaria and consult an Infectious Disease specialist. Illness from its cousin species P. vivax can be delayed up to 3 years from travel, but P. vivax malaria is much milder, never fatal. We worry most about P. falciparum in travelers back ≤40 days.
Other bad viruses can be imported from the tropics, like Ebola (bleeding), Dengue (bleeding; “bone pain”), Chikungunya (severe joint pains), and more. We consult a specialist as needed, but Malaria is still the most common, quickly lethal, & easily treatable.