Warts are caused by human papillomavirus, a family of viruses with more than 100 different strains that can affect both genital and nongenital areas of the body. Roughly 60% of these strains trigger nongenital warts, often on hands and feet. The virus amplifies the output of tissue-generating cells, which produce thick, knobby growths. The immune system generally keeps HPV in check during adulthood, but as immunity decreases with age, warts may become more common. The growths often clear up on their own, but since HPV is transmissible and the warts can spread, it's best to deal with them as soon as they're visible.