Warts are small growths on the skin that are caused by the human papilloma virus (HPV). HPV causes hard protein (keratin) to grow in the upper layer of your skin and form small, irregular bumps that are rough to the touch. There are over 100 types of HPV, and different strains of the virus are responsible for different types of warts.
The viruses that cause warts are usually completely harmless. Some of the most prevalent types of warts are common warts, plantar warts and plane warts. Not everybody who comes into contact with wart viruses becomes infected. The reasons for this are not completely understood but your genetic makeup and immune system are likely involved. A family history of warts could make you more susceptible to the virus, and people who have a weakened immune system are also at higher risk.