The human papilloma virus (HPV) is a family of more than 150 spherical microbes that may look beautiful under a microscope but can be ugly and deadly when they infect the human body. About 40 HPV strains are passed from person to person through sexual activity. A handful of “high-risk” strains can cause persistent infections that morph into cancers of the cervix, penis, anus, vulva, vagina and throat.

The best protection? The HPV vaccine. Experts say this three-shot vaccine could prevent 21,000 HPV-related cancers a year. As a result, it might save lives — 4,000 women die each year from cervical cancer. Yet millions of preteens, teens and young adults are missing out.

According to the Centers for Disease Control and Prevention (CDC), 42 percent of teen girls and 65 percent of teen boys eligible for the HPV vaccine hadn’t received a single dose as of June 2014. Parents whose children didn’t receive it told researchers in one recent study that they were worried about safety or didn’t think their son or daughter needed it.

Here are nine things you should know about this important yet misunderstood vaccine.

1. The HPV vaccine protects against viruses that cause 70 percent of cervical cancers. Two vaccines are available. Gardasil helps protect against four HPV strains: 6 and 11, which cause genital warts, and 16 and 18, which cause most cervical cancers. Given to girls, young women, boys and young men, Gardasil may also protect against some cancers of the vagina, vulva and anus. Cervarix protects against HPV types 16 and 18 and is for girls and young women.

2. It works. It can take many years for an HPV infection to trigger the development of cervical cancer. Now, studies are beginning to show that the vaccine stops this process. In a recent Danish study of more than 450,000 young women, the risk for precancerous cell changes of the cervix— a last step on the road to cervical cancer — was reduced by 60 to 80 percent in those who had received the vaccine up to six years earlier. 

3. Protection lasts at least eight years. The longest-running studies have found that protection keeps on going — eight years at last count according to the National Cancer Institute (NCI), and the CDC says there is no evidence of weakened protection over time. The NCI says more will be known about how long the vaccine provides immunity as more time passes and more research is conducted.

4. It’s safe. The safety of the HPV vaccine is on par with that of other childhood and teen vaccines according to the Food and Drug Administration and the CDC. Gardasil may cause more fainting than other vaccines. That’s why healthcare practitioners ask vaccine recipients to remain seated for 15 minutes after the shot. Early reports suggested the vaccine might increase the odds for blood clots, but the Danish study of a half-million women who were vaccinated found no extra risk.

5. If you’re sexually active, you can’t avoid these viruses. Almost 100 percent of sexually active adults will have an HPV infection at some point, the CDC says. Every year, 17,500 women and 9,300 men are diagnosed with HPV-related cancers.

6. Best vaccination age is 11 or 12. The CDC recommends the three-dose HPV vaccine for preteens at age 11 or 12. (It may be given as early as age 9.) The vaccine triggers a stronger immune response in preteens than in teens or young women. And it’s best to build immunity to these viruses before sexual activity begins. If you or your child missed those target ages, it’s not too late. Women can receive the vaccine through age 26, men through age 21.

7. Getting the vaccine doesn’t send teens the wrong message. Wondering whether teens who get the vaccine will see it as a green light to have sex? You’re not alone. So did researchers at Cincinnati Children’s Hospital Medical Center. They interviewed 300 young women, ages 13 to 21, and found that vaccinated teens didn’t start having sex any sooner than others. And they knew that the vaccine didn’t protect against other sexually transmitted diseases.

8. Yes, it hurts a little. But you need all three shots. Despite what you may have read online or heard from friends or the media, research refutes claims that these shots are extra-painful. A 2009 University of North Carolina at Chapel Hill study asked 889 parents about the experiences of their daughters who had received the vaccine. The result: The HPV vaccine was less painful than a tetanus booster shot or meningococcal vaccine. Few felt dizzy and none fainted. And pain didn’t keep families from returning so their daughters could receive all three shots. (The second shot is given one to two months after the first; the third is given six months after the first.)

9. Vaccinated women still need cervical-cancer checks. No vaccine is perfect. Women should still have Pap tests and HPV tests to check for early signs of cervical cancer, the American Cancer Society recommends.

Sari Harrar is an award-winning health, medicine and science journalist whose work appears in Dr. Oz The Good Life magazine, Good Housekeeping, O--Oprah Magazine, Organic Gardening and other publications.