Yellow fever is an illness which is caused by a virus. The illness can cause bleeding problems. It is called yellow because the skin sometimes becomes yellow, like it does with jaundice. There is a vaccine which can stop the disease, but many people in Africa and South America are not vaccinated against it. The World Health Organization say that 200,000 people are made ill with yellow fever every year, and that 30,000 people die from it.
Yellow fever is spread through the bite of an infected mosquito. It does not spread from person to person by direct contact.