Let’s get something straight: you can’t tell if someone has a sexually transmitted disease (STD) just by looking at them. Many STDs do not come with visible symptoms, so the only way to find out is to get tested. We highly recommend you do this at least once a year, and the sooner, the better. If left untreated, STDs can cause serious damage to your health ranging from chronic pain, an increased risk of certain cancers, and infertility.

Reference: http://www.bustle.com/articles/38858-5-things-every-college-student-should-know-about-sex/