Yaws is a tropical infection of the skin, bones, and joints caused by the spirochete bacterium Treponema pallidum pertenue. The disease begins with a round, hard swelling of the skin, 2 to 5 cm (0.79 to 1.97 in) in diameter. The center may break open and form an ulcer. This initial skin lesion typically heals after 3–6 months. After weeks to years, joints and bones may become painful, fatigue may develop, and new skin lesions may appear. The skin of the palms of the hands and the soles of the feet may become thick and break open. The bones (especially those of the nose) may become misshapen. After 5 years or more, large areas of skin may die, leaving scars.

Yaws is spread by direct contact with the fluid from a lesion of an infected person. The contact is usually of a nonsexual nature. The disease is most common among children, who spread it by playing together. Other related treponemal diseases are bejel (T. pallidum endemicum), pinta (T. carateum), and syphilis (T. p. pallidum). Yaws is often diagnosed by the appearance of the lesions. Blood antibody tests may be useful, but cannot separate previous from current infections. Polymerase chain reaction is the most accurate method of diagnosis.

No vaccine has yet been found. Prevention is, in part, done by curing those who have the disease, thereby decreasing the risk of transmission. Where the disease is common, treating the entire community is effective. Improving cleanliness and sanitation also decreases spread. Treatment is typically with antibiotics, including: azithromycin by mouth or benzathine penicillin by injection. Without treatment, physical deformities occur in 10% of cases.

Yaws is common in at least 13 tropical countries as of 2012. Almost 85% of infections occurred in three countries—Ghana, Papua New Guinea, and Solomon Islands. The disease only infects humans, although 18th century French historian Pierre François Xavier de Charlevoix describes it as a disease among geese in the Antilles. Efforts in the 1950s and 1960s by the World Health Organization decreased the number of cases by 95%. Since then, cases have increased, but with renewed efforts to globally eradicate the disease by 2020. In 1995, the number of people infected was estimated at more than 500,000. In 2016, the number of reported cases was 59,000. Although one of the first descriptions of the disease was made in 1679 by Willem Piso, archaeological evidence suggests that yaws may have been present among human ancestors as far back as 1.6 million years ago.