Sexually transmitted diseases (STDs) can also be known as sexually transmitted infections (STIs). They are infections that are usually spread from person to person during vaginal, anal, and oral sex. Although many STDs are quite common, a lot of infected people do not show any symptoms, and are unaware that they have an STD and can pass the disease to others. Without treatment, STDs can lead to serious health complications. However, most STDs are easy to test for and can be effectively treated.