The human papilloma virus comes in more than 100 varieties, from the innocuous strains that cause the common wart to the approximately 30 types that live in the genital tract, including about 10 to 15 that can cause cancer. HPV is a persistent and stealthy invader. Because it can live in the cells of the outer skin, it eludes safe-sex practices. And since it rarely produces symptoms, few people with the virus know they have it. In fact, up to 75 to 80 percent of sexually active women worldwide are infected at some point.
Most infections resolve spontaneously.
A small fraction, though, develop into cervical cancer. Cervical cancer is the number two killer of women in the world, with approximately 500,000 cases annually. It s a disease mostly of the poor and disenfranchised, women with no access to the Pap smears that detect cervical abnormalities. On their surfaces, HPV cells have well-defined antigens, tiny bits of protein that differentiate them from normal cells, red flags that prompt the immune system to attack. These antigens, called E6 and E7, are known quantities, a dream come true, Wu thought, for the tumor immunologist who could aim a cancer vaccine directly at them.