Yellow fever spreads mainly through direct contact with patients, clothing, bedding, and other contaminated objects.

Yellow fever often spread through crowded cities, so it was natural to suspect the same things feared in many other contagious diseases: sick patients, their rooms, clothing, bedding, and other contaminated objects.

Such contaminated objects were often treated as fomites.

Cuban physician Carlos Finlay proposed a very different mechanism in the nineteenth century. He argued that mosquitoes transmitted yellow fever from one person to another, but the idea was not widely accepted at first.

Around 1900, the U.S. Army Yellow Fever Commission tested the competing explanations.

In one set of experiments, volunteers entered rooms containing bedding, clothing, and other materials heavily contaminated by yellow-fever patients. Despite close contact with these objects, they did not develop the disease.

Mosquito experiments produced the opposite result. When mosquitoes had first fed on yellow-fever patients and were later allowed to bite susceptible volunteers, transmission could occur.

The results redirected prevention toward mosquito control.

Yellow fever is now known to be caused by a virus. In urban outbreaks, Aedes aegypti is the main vector for transmission between people.

The ecology is broader outside cities, where other mosquito species and animal hosts take part in forest transmission cycles. The historical correction is that contaminated clothing and bedding were not the principal explanation for urban yellow-fever spread.