Yaws is caused by the spirochete Treponema pertenue, which penetrates an area of broken skin, such as an insect bite, scrape, cut, or abrasion. The initial lesion, called the mother yaw, is a painless ulceration that takes 2 to 8 weeks to grow. It contains a large amount of spirochetes that can be transmitted from person to person via skin contact. In late yaws, the lesions are no longer contagious.

Complications of yaws include secondary bacterial infection and scarring. Similar to tertiary syphilis, bone, cartilage, skin, and soft tissue may be affected in untreated patients after 5 to 10 years. Resulting conditions may include gangosa (destructive ulcerations of the nasopharynx, palate, and nose); painful skeletal deformities, especially of the legs (termed saber shins); and other soft-tissue changes (gummas, inflammatory cell infiltration).

Yaws has four stages. In the primary stage, a large bump lesion develops up to 3 weeks after inoculation. Called the mother yaw, this lesion is also known as buba, buba madre, or primary frambesioma. It is slightly elevated with a crust that sheds, leaving an area that resembles the texture of a raspberry or strawberry. The lesion is painless, although lymph nodes in the area may be swollen. A light-colored scar remains when the lesion heals, usually in 2 to 9 months. Bone and joint pain and swelling may occur in early disease. Other symptoms include fatigue, general malaise, and anorexia.