There's a lot of misinformation floating around about STDs these days, and it's a subject no one likes to broach with their teenager. Regardless of how uncomfortable it may be, it's a conversation that needs to happen, since teens who are sexually active are at high risk for an STD.

The Federal Centers for Disease Control estimates that some 19 million new infections occur annually, and almost half of them hit young people between the ages of 15 and 24. About half of teenage girls living in cities in the United States acquire at least one of three common STDs--chlamydia, gonorrhea or trichomoniasis--within two years of starting to have sex, according to a new study reported by HealthDay News. The study also found that it was common for these girls to come down with repeated infections. Some 381 girls, aged 14 to 17, participated in the research.

Screening for an STD may not actually start for several years after a girl becomes sexually active, the study found, and this is especially true for those who became sexually active at a young age.

"This is important because many clinicians are reluctant to address sexual activity with younger teens, and may miss important prevention opportunities," Indiana University School of Medicine pediatrics professor J. Dennis Fortenberry, MD, senior author of the study, said in HealthDay News.

An important part of preventing infection is knowing what's true and what isn't. Here are some of the most common myths surrounding sexually transmitted diseases.

Myth: You know you have an STD because you have symptoms.

Fact:  "A sexually transmitted disease can affect anyone who is sexually active, and you don't necessarily have to have symptoms," says Hal Danzer, MD, fertility specialist and co-founder of the Southern California Reproductive Center.  "You can be an asymptomatic carrier." In this case, you would not be able to warn a sexual partner that you were infected, and you could potentially transmit the STD to him or her,  Dr. Danzer explains.

Myth: You can't get an STD if you have sex with someone just once.

Fact: Many STDs are very easy to transmit, explains William McCormack, chief of infectious diseases at the State University of New York, Downstate Medical Center in Brooklyn. "Gonorrhea and chlamydia are especially easy to transmit," he says. "From a man to a woman, there is about a 50 percent chance of getting gonorrhea after just one exposure. From a woman to a man, there is about a 20 percent chance after one exposure."

Myth: All STDs respond to antibiotics.

Fact: While certain STDs such as syphilis and gonorrhea are bacterial and respond to antibiotics, others do not, Dr. McCormack explains. Herpes is caused by a virus so it is treated symptomatically. HIV is a virus, too, which means that there is no cure.

Myth: You can't get an STD by having oral sex.

Fact: Oral herpes can be transferred to the genital area, explains Danzer, and the virus is extremely common. In fact, he says, between 25 and 30 percent of young people have herpes.

Myth: If I get the HPV vaccine, I don't have to worry about getting an STD.

Fact: The Centers for Disease Control recommends the vaccine for girls as young as age 11, with catch-up shots up to age 26. It's most effective when given before a girl becomes sexually active and might already have contracted HPV (human papillomavirus), which is the virus that the vaccine blocks. But the vaccine does not protect against all STDs.

The best protection against STDs, says McCormack, is having a single, long-term sexual partner. Other than that, he says, "Prevention is accomplished by carefully selecting one's partners and by using condoms."