In the United States alone, there are nearly 20 million cases of new sexually transmitted infections yearly, from just eight viruses and bacteria, according to data from the Centers for Disease Control and Prevention (CDC).

The eight most common STDs in the U.S. are chlamydia, gonorrhea, hepatitis B virus (HBV), genital herpes, HIV, human papillomavirus (HPV), syphilis and trichomoniasis. About 50.5 million of these current infections are in men, and 59.5 million are in women, according to the CDC’s 2013 report, in which the researchers looked at 2008 data.

Each year, new cases of STDs cost nearly $16 billion in direct medical costs. Fifty percent of these new infections occur in young people, ages 15-24, even though this age group represents only a quarter of people who have had sex.

 

Gonorrhea is the second most commonly reported STD in the United States, and it is especially common among people ages 15 to 24, according to the CDC. Gonorrhea can affect the genitals, rectum and throat, and can be cured with the right medication. Left untreated, the bacterial infection can lead to significant health problems, such as infertility and chronic pelvic pain.

It is estimated that more than 800,000 new infections occur each year. In 2012, there were 334,826 cases reported in the United States, which is a rate of 107 cases per 100,000 people. Rates by state ranged from 8 per 100,000 in Wyoming, to 230 in Mississippi and 389 in the District of Columbia per 100,000 people.

Although syphilis was nearly eliminated from the United States about a decade ago, the number of cases is on the rise again, according to a 2014 report from the CDC.

Syphilis is easy to cure in its early stages, but can cause long-term problems if the bacterial infection is left untreated. Symptoms of syphilis can vary depending on the stage of the disease. The first two stages, during which the condition is most contagious, are known as primary and secondary syphilis.

Many cases of STDs — such as chlamydia, gonorrhea and syphilis — continue to go undiagnosed and unreported, and data on several other infections — such as HPV, herpes and trichomoniasis — are not routinely reported by medical centers to the CDC. As a result, the CDC’s yearly reports on sexually transmitted infections captur