Sexually transmitted diseases (also known as STDs - or STIs for "sexually transmitted infections") are infectious diseases that spread from person to person through intimate contact. STIs can affect guys and girls of all ages and backgrounds who are having sex - it doesn't matter if they're rich or poor. STIs are more than just an embarrassment. They're a serious health problem. If untreated, some STIs can cause permanent damage, such as infertility (the inability to have a baby) and even death (in the case of HIV/AIDS).
Some of the things that increase a person's chances of getting an STI are: