Many people who have a sexually transmitted disease may not know it.

In 2007, nearly one-third of the people diagnosed with HIV likely had it for a long time without knowing it because they developed AIDS less than a year after their HIV test. Meanwhile, they may have infected others before they knew they were infected, according to the Centers for Disease Control and Prevention.

Within a few weeks of being infected with HIV, some people develop flu-like symptoms that last for a week or two, but others have no symptoms at all, the CDC said. Medications can limit or slow down the destruction of the immune system, improve the health of people living with HIV and reduce their ability to transmit HIV. People can live perhaps for decades before they develop AIDS, the late stage of HIV infection. There is no cure.

An estimated 56,300 Americans are infected with HIV each year — an average of one new infection every 91/2 minutes, according to the CDC.

Without treatment a person infected with HIV will develop AIDS in about 10 years. With early treatment a 25-year-old adult can survive on average 39 more years, the CDC said.

Chlamydia is the most commonly reported STD in the United States. Most people who have chlamydia don't know it since the disease often has no symptoms.

It's easy to cure with antibiotics, but it can impact a woman's ability to have children if left untreated. Women whose sex partners have not been appropriately treated are at high risk for re-infection, the CDC said.

In Virginia, there were 21/2 times as many women diagnosed in 2009 as men: 22,752 women compared to 8,393 men, according to the state health department.

Slightly more women than men were diagnosed with gonorrhea in Virginia in 2009 — 4,329 women compared to 3,470 men.

Gonorrhea can sometimes be treated with antibiotics, but drug-resistant strains are increasing. Men may experience no symptoms, or they may experience a burning sensation when urinating, a discharge from the penis or painful, swollen testicles. Symptoms in women may be mild, including pain while urinating, increased vaginal discharge or vaginal bleeding between periods. It can cause infertility in men and impact a woman's ability to have children if left untreated. Gonorrhea can affect the anus, eyes, mouth, genitals or throat, and it may be contracted again.

Symptoms of syphilis include a firm, round, small and painless sore on the genitals, anus or mouth, or a rash on the body, especially on the palms of the hands or the soles of the feet. In 2008, 63 percent of the reported syphilis cases were among men who have sex with men, and the rates of syphilis increased the most between 2004 and 2008 among 15- to 24-year-olds.

A single injection of penicillin, an antibiotic, will cure a person who has had syphilis for less than a year. Additional doses are needed to treat someone who has had syphilis for longer than a year. Treatment will not repair damage already done. After treatment, a person must abstain from sexual contact with new partners until the syphilis sores are completely healed. Those with syphilis must notify their sex partners so that they also can be tested and receive treatment if necessary, the CDC said.