Flat warts (also known as verruca plana and plane warts) are 1-3 mm, round or oval, slightly raised, smooth papules induced by human papillomavirus (HPV) types 3, 10, 28, and 49. Flat warts are commonly painless and can be yellowish-brown, skin-colored, or pink. They may appear singularly, in clusters, or in a linear arrangement. Flat warts typically present on areas of the body that have contact with other people and objects, such as the face, arms, hands, and feet; however, they can appear anywhere on the body.

The warts arise from benign strains of HPV and are not known to cause cancer. They are contagious and spread easily over the body. Transmission is commonly via person-to-person contact or via fomites. Existing skin trauma (ie, cuts, scratches, burns, eczema) predisposes patients to contracting HPV. A person with flat warts may spread the warts to a different part of the body (autoinoculation) through trauma to the skin such as scratching or shaving.

Children, young adults, and immunocompromised patients are most susceptible. Widespread or extensive warts are often presenting signs of an immunocompromised state. Warts may be more numerous and more difficult to treat in immunocompromised patients. Excessive sun exposure can lead to exacerbation.

While warts are normally self-limited in children, they may be difficult to treat in adults. Longer periods of treatment are usually necessary for adults.