A virus that lives in the flat cells on the surface of the skin and on the moist mucosal membranes in the body. These include the urethra (peehole), vagina, cervix, penis, anus and throat. There are at least 120 types or strains of human papilloma virus. While many of the HPV strains cause no symptoms, others can cause warts, like the warts people get on their hands and feet, or warts on their genitals, anus and thighs. Some of the HPV strains can cause cancer. There are now very effective vaccines for HPV.