Gardasil, the new vaccine against human papilloma virus (HPV) -- which causes cervical cancer and genital warts -- is now available nationwide.

Here is what you need to know now about this new vaccine.

1. What is Gardasil?

Gardasil is a vaccine, licensed for use in June 2006, by the FDA. It targets four strains of human papillomavirus (HPV) -- HPV-6, 11, 16, and 18. HPV-16 and HPV-18 account for about 70% of all cervical cancers. HPV-6 and -11 cause about 90% of genital warts. HPV is also linked to anal cancer.

2. How does HPV spread?

Sexual activity spreads the virus, a very common one. It's one of the most common sexually transmitted diseases (STDs) in the country, according to the CDC, with more than 20 million people currently infected and another 6.2 million contracting the virus each year.

About half of those with HPV are aged 15 to 24. Surveys suggest 3.7% of U.S. girls have sex by age 13, and 62.4% have had sex by the 12th grade.

3. Who should get the vaccine?

Gardasil is approved by the FDA for girls and women ages 9 to 26. The CDC and the American Academy of Pediatrics recommend that the vaccine be given routinely to girls at age 11 to 12 years old, although doctors may choose to vaccinate girls as young as 9. The CDC also recommends the vaccine for women age 13 to 26 who did not receive the vaccine at an earlier age.

However, if a girl or woman is already infected with HPV, the vaccine will not prevent that strain of HPV from causing disease. It will protect against new infections with other strains of HPV included in the vaccine.

The vaccine is also being studied in women up to age 45, although that group may be targeted for the vaccine later.

The vaccine is being studied in males, too. Men can get HPV infections and can pass the virus to their sex partners. HPV causes genital warts and is associated with rare cases of cancer of the penis. Particularly in gay men, HPV is linked to anal cancers. Merck is currently testing Gardasil in men, including gay men.