Yellow fever occurs in tropical South America and Africa and among travelers to those areas. It is caused by a virus transmitted to humans by the bite of an infected mosquito. This virus can cause significant illness that may progress to major organ damage, particularly of the liver. It may cause hepatitis, an inflammation (cellular damage) of the liver. Hepatitis can cause jaundice, a yellow discoloration of the eyes and skin that led to the name "yellow fever." This liver damage can also lead to disruption of the normal clotting of blood, resulting in bleeding. The bleeding can manifest as the so-called black vomit of yellow fever. A vaccination against this infection is required for those who travel to endemic areas (geographic regions where exposure is possible), such as central Brazil and Peru in South America and Ghana and Liberia in West Africa. Most people can receive this vaccination from their local department of health or from a travel medicine clinic as long as there are no medical reasons not to receive the vaccine. The August 27, 2008, issue of JAMA includes a Commentary on a classic 1901 JAMA article by Walter Reed, one of the pioneers in research on the cause of yellow fever.