Infectious diseases seem to come and go, sometimes causing scary outbreaks, while other times seeming to disappear. But some infectious pathogens are always with us, lurking just below the surface of society.

Sexually transmitted diseases are one major group of diseases that make for ongoing hidden epidemics: 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). 

Perhaps 20 million new cases sounds like a high number. But because infections can persist for a long time, especially in people who aren't aware they are infected, the number of existing infections at any given time is even higher, at 110 million, according to CDC researchers. [See maps of the US with STD rates by state]

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.

There are large differences in how commonly these STDs occur. Chlamydia and HPV are the two most common newly acquired sexually transmitted infections affecting millions of people each year, whereas new infections with HIV and hepatitis B occur in less than 50,000 people yearly.

For some diseases, such as chlamydia, the number of new infections in a given year is higher than the number of existing infections. That's because the new infections that are diagnosed and treated are not counted in the estimate of existing infections, said Elizabeth Torrone, a researcher at the CDC's center for STDs. On the other hand, viruses such as HIV cause life-long infections, and so the number of existing infections is much higher than the number of new cases yearly.

The most current data estimates that about 1.8 million people have chlamydia, as opposed to 1.6 million in the previous study. But because estimates have a margin of error, the current rate should not be interpreted as an increase, Torrone told Live Science. 

Chlamydia

Chlamydia is the most commonly reported STD in the United States. But most people with chlamydial infections may not show any symptoms, and so the number of actual infections is much higher than the number of those reported, which was 1.4 million in 2012, or a rate of 457 cases per 100,000 people.

It is easy to cure chlamydia – it is a bacterial infection treated with antibiotics. If left untreated, the infection can make it difficult for a woman to get pregnant. An untreated chlamydial infection can cause pelvic inflammatory disease (an infection of the reproductive organs), in about 10 to 15 percent of infected women, and lead to infertility. [7 Devastating Infectious Diseases]

Rates of chlamydial infection differ by state. In 2012, chlamydia rates ranged from 233 cases per 100,000 people in New Hampshire to 774 in Mississippi and 1,102 in the District of Columbia per 100,000 people.

Gonorrhea

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.

Syphilis

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.

In 2012, the 15 states and the District of Columbia that had the highest rates of primary and secondary syphilis accounted for 70 percent of all U.S. cases. The national rate of syphilis is currently estimated at 5 cases in every 100,000 people.

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 capture only a fraction of the true burden of STDs in America, the CDC says.

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