Yaws is the most prevalent infectious, nonvenereal treponemal disease and is caused by Treponema pallidumpertenue. Yaws, endemic syphilis (bejel), and pinta collectively constitute the endemic treponematoses. Yaws is transmitted by direct skin contact and primarily affects children younger than 15 years, with a peak incidence in those aged 6-10 years. Similar to syphilis, yaws can persist for years as a chronic, relapsing disease. [1, 2, 3]
Yaws continues to be endemic along the tropical belt in areas characterized by hot temperatures, high humidity, and heavy rainfall. These conditions, coupled with the persistence of poverty, poor sanitation, overcrowding, and lack of public health surveillance, allow for yaws perpetuation. 
Between the years 1952 and 1964, the World Health (WHO) and UNICEF (United Nations Children’s Fund) undertook a major worldwide campaign to eliminate the endemic treponematoses by treating 300 million people in 46 countries with benzathine benzylpenicillin. They achieved a 95% success rate; however, there was a reemergence of yaws in the 1970s. In 1995, the WHO estimated that there were 460,000 infectious cases of yaws throughout the world, with 400,000 in western and central Africa, 50,000 in Southeast Asia, and the remainder in other tropical areas. [3, 5, 6, 7, 8]
A new yaws eradication program was proposed in 2012 by the WHO following a study that showed that oral azithromycin can successfully treat yaws in rural, tropical areas. Compared to benzathine benzylpenicillin, oral azithromycin is a simpler regimen that does not require trained medical personnel for administration.  In India, yaws was successfully eradicated through a programm based on providing information to the population at risk, screening, and treatment. The WHO has concluded that this new eradication campaign can completely eliminate yaws worldwide by 2020. 
Etiology and Pathophysiology
Yaws is caused by Treponema pallidumpertenue, a slender spirochete that is serologically indistinguishable from the spirochete T pallidum, which causes syphilis. As with the other nonvenereal treponematoses, yaws is not found in urban centers, is not sexually transmitted, and is not congenitally acquired. The major route of infection is through direct person-to-person contact, and the treponemes associated with yaws are located primarily in the epidermis. Children serve as the primary reservoir for yaws, spreading the disease via skin-to-skin and skin-to–mucous membrane contact.
During the incubation period, T pallidum pertenue invades the subcutaneous lymphatics and disseminates hematogenously. The ulcerative skin lesions that develop early in the disease course are teeming with spirochetes, which can be transmitted via direct skin-to-skin contact and via breaks in the skin due to trauma, bites, or excoriations. Agmon-Levin et al suggested that the antitreponemal antibodies that build up in certain populations may also be protective for atherosclerosis while also being pathogenic for yaws. 
Yaws, like syphilis, has been classified into the following four stages:
- Primary stage, in which the initial yaws lesion develops at the inoculation site
- Secondary stage, in which widespread dissemination of treponemes results in multiple skin lesions that are similar to the primary yaws lesion
- Latent stage, in which symptoms are usually absent but skin lesions can relapse
- Tertiary stage, in which bone, joint, and soft tissue deformities may occur
Cutaneous lesions characterize the primary and secondary stages of yaws. The tertiary stage of yaws may involve the skin, bones, and joints.
Another classification distinguishes early yaws from late yaws. Early yaws includes the primary and secondary stages and is characterized by the presence of contagious skin lesions. Late yaws includes the tertiary stage, when lesions are not contagious.
Yaws does not occur in the United States. However, according to the last estimate by the World Health Organization (WHO), in 1995, the prevalence of endemic treponematoses, mostly yaws, was 2.5 million, with 460,000 cases being infectious. In 2006, India declared that yaws had been eliminated in that country. According to the WHO, in 2010, yaws continues to be common in areas with the poorest population; the endemic nations included Indonesia, Timor-Leste, Papua New Guinea, the Solomon Islands, Vanuatu, Benin, Cameroon, Central Africa Republic, Congo, Cote d’Ivoire, Democratic Republic of the Congo, Ghana, Sierra Leone, and Togo. [3, 5, 6, 7, 8]
The population at risk of contracting yaws worldwide is estimated to be 34 million, evenly distributed between men and women (17 million each). Children serve as the primary reservoir for yaws, as the condition is transmitted from person to person via direct contact. By age group, the population considered to be at risk includes 23 million who are 14 years of age or younger and 11 million who are between the ages of 16 and 24 years. Approximately 75% of those affected by yaws are children younger than 15 years, with the peak incidence occurring between the ages of 6 and 10 years. [3, 5, 6, 7, 8]
Unless treated, yaws can become a chronic, relapsing disease after 5-15 years, with skin, bone, and joint involvement. In most patients, yaws remains limited to the skin, but early bone and joint involvement can occur. Although yaws lesions disappear spontaneously, secondary bacterial infections and scarring are common complications.
In 10% of yaws cases, patients enter a late stage (tertiary stage) characterized by destructive cutaneous lesions and severely deforming bone and joint lesions. Tissue damage occurring in late yaws is irreversible. Neurologic and ophthalmologic involvement may also occur. Relapses may occur at intervals of up to 5 years after infection.