STDs (sexually transmitted diseases) are very common, with over 2 million new cases reported every year in the US alone. They can be transmitted through oral, anal and vaginal sex, and can even be spread without sexual relations, such as sharing needles. In some cases, people may not experience more than mild symptoms, and some do not show symptoms at all. Asymptomatic cases can make it difficult for a person to know that they have an STD without receiving a test.