1. Cancer connection: There are more than 100 types of Human papillomaviruses or HPV, the most common sexually transmitted infection in the United States. Most are harmless, but about 30 types put you at risk for cancer. Almost all women will have HPV infections at some point, but very few will develop cervical cancer; their immune systems will usually suppress or eliminate HPVs, says the Centers for Disease Control and Prevention. Only HPV infections that do not go away over many years lead to cervical cancer.

2. How you get it: Through skin-to-skin sexual contact with an infected partner. Transmission by genital contact without intercourse is not common, but it has been reported. Oral-genital and hand-genital transmission of some HPV types is possible, says the American Cancer Society.

3. How common: Infection is very common soon after a woman becomes sexually active. In one recent study, more than 50 percent of college-age women were found to have acquired an HPV infection within four years of first having sex, says the American Cancer Society.

4. Prevention: Abstinence or a long-term, mutually monogamous relationship with an uninfected partner, although it's difficult to tell if a partner who has been sexually active in the past is infected. Correctly using latex condoms greatly reduces the risk, but it doesn't completely protect because areas not covered by a condom. A new vaccine called Gardasil is recommended for 11- and 12-year-old girls before their first sexual contact. Another promising vaccine, Cervarix, is being tested, but hasn't been approved yet. Studies suggest the vaccines can protect against some HPV for at least four years; the need for a booster vaccine is being researched. A controversial Florida bill that would have required all sixth-grade girls to be vaccinated died in the Florida Legislature last year.

5. Guys: HPV also can cause genital warts, penile and anal cancer. It now causes as many cancers of the upper throat as tobacco and alcohol, probably due both to an increase in oral sex and the decline in smoking, researchers say. A vaccine for boys may soon become available, which also would help prevent men from spreading the virus to women.