Dengue fever is a viral infection that, like many tropical diseases—including malaria and yellow fever—is transmitted by mosquitos. Dengue isn’t caused by just any old mosquito, but by a species known as aedes that breed in stagnant water, usually in densely populated urban areas with improper drainage. Plant containers and abandoned rubber tires are particularly common breeding grounds.

Dengue mosquitos usually attack during the daytime and are most common during hot, humid rainy periods. In recent years, Rio de Janeiro has had large dengue epidemics during its rainy summer months. Although rarely fatal, dengue is like having a really debilitating case of flu. Symptoms include fever, aching muscles, headaches, nausea, weakness, vomiting, and a rash. In general, the worst symptoms last 5–7 days, but full recuperation can take longer.

Diagnosis is via a blood test. There is no vaccine for dengue, nor is there treatment aside from rest, plenty of liquids, and acetaminophen (Tylenol). Do not take aspirin. Only severe cases require hospitalization. The best thing you can do to avoid infection is to take precautions to avoid getting bitten in the first place (see Mosquitos).