Warts are the most common type of skin infection. They are infections of the skin with the human papilloma virus. There are more than 100 different strains of the human papilloma virus. Usually warts start growing after infection with one or more strains of HPV. The virus makes the top layers of the skin grow rapidly, creating the different non-cancerous skin growth. It can be difficult to tell warts apart from other skin conditions, such as skin tags, moles and actinic keratosis.