Yaws is an infectious disease that occurs in the tropics. It is caused by the bacteria Treponema pallidum. This is the same bacteria that causes syphilis but yaws is caused by a slightly different version of the bacteria and is not sexually transmitted. The disease affects the skin and bones and occurs in three stages. The last stage is the most severe and results in significant facial disfiguration. However, only ten percent of cases progress to this stage. The number of cases of the disease has decreased dramatically since the 1950's.