Statistics show that 50% of all Americans will be diagnosed with an STI or STD by the time they reach adulthood. So that shows you in some way how common that STI's and STD's are. The most common STI or STD in America is Chlamydia and the second most common is HPV or Human papilova virus. Chlamydia is very common, it is asymptomatic in most people which means there are no symptoms. So the only way to know that you have it is to be tested. It is completely curable and treatable with antibiotics. The really important thing to know about Chlamydia is if left untreated it can cause infertility in both men and women. In women it can lead to public inflammatory disease or PID and in men it can lead to inflammation of the apedenunas the tubes through which the sperm will shoot out from the body. HPV is human papilova virus also known as the genital warts is also a very common sexually transmitted infection and that is caused by a virus. It is treatable but not necessarily curable. It is very transmissible from skin to skin top contact. The best way to actually prevent it is to have a barrier between your skin and someone else's skin. Someone may transmit HPV even if they don't have symptoms but the things to look out for are the warts or the raised bumps on the shaft of the penis, on the outside of the vulva or around the rectum, sometimes those look like warts, sometimes they are flat, sometimes they are actually not visible to the naked eye and you need to have your provider check in a different way. Sometimes they squirt a little bit of vinegar into the vagina , around the vulva or on the shaft of penis, around the anus in order to be able to see the HPV and they sometimes take it off in some way or give you a cream or medication and sometimes HPV just goes away by itself . So again the two most common sexually transmitted diseases or infections in America are Chlamydia and HPV.