The disease is transmitted by skin contact with infected individuals, the spirochete entering through an existing cut or similar damage. Within ninety days (but usually less than a month) of infection a painless but distinctive 'mother yaw' ulcerous papule appears on the skin at the point of entry, it is often described as raspberry-like and is 10-50 mm in size. This lesion will persist for up to nine months and other secondary growths will appear on the body as the original one heals, there may also be inflammation of the fingers (dactylitis).
If untreated a secondary stage occurs after up to four months of latency, it is marked by more 'raspberry' growths but smaller and ulcerous - exuding a thin, highly infective fluid which attracts flies. These growths may also merge together into thick fissured plaques, which can occur on the feet and induce a distinctive gait. These secondary growths are irreversible but there can be relapsing lesions and asymptomatic periods.
In 10-20 % of cases the disease can progress over a decade or more to a tertiary stage with destructive lesions of the skin and bones. Large subcutaneous nodules develop and grow before abscessing and ulcerating, these can become infected and may merge together forming serpiginous tracts. These tracts heal with keloid formation which can cause deformities, disabilities and limb contractures. The bone lesions caused are periostitis, osteitis, and osteomyelitis, damage to the tibia can lead to a condition known as sabre shins. In a very few cases a condition known as goundou is caused where growths on the nasal maxillae can result in extensive and severe damage to the nose and palate.
The largest group afflicted by Yaws are children aged 6 to 10 years in the Caribbean Islands, Latin America, West Africa, India, Oceania or Southeast Asia. There were WHO-funded campaigns against yaws from 1954 to 1963 which greatly reduced the incidence of the disease, although more recently numbers have risen again.
The disease is identified from blood tests or by a lesion sample through a darkfield examination under a microscope. Treatment is by a single dose of penicillin, erythromycin or tetracycline, recurrance or relapse is uncommon.
Examination of ancient remains has led to the suggestion that yaws has affected hominids for the last 1.5 million years. The current name is believed to be of Carib origin, "yaya" meaning sore; frambesia is from the French for "raspberry".