Yellow fever is a mosquito-borne virus found in parts of Africa and South America. It infects the liver and blood vessels, causing fever, jaundice, bleeding, and in severe cases, organ failure. In the past, outbreaks killed thousands in cities like Philadelphia and Rio de Janeiro. Today, a single vaccine dose provides long-term protection and is required by some countries for entry. Vaccination campaigns in high-risk areas have sharply reduced deaths.

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