Sexually Transmitted Infections (STIs) are some of the most widespread infections in the world. There are at least 20 different infections that are caused by viruses, bacteria and single-celled organisms. Exposure to an STI can occur any time you have sexual contact with anyone that involves the genitals, the mouth (oral), or the rectum (anal). Bacterial STIs can be treated and cured, but STIs caused by viruses usually cannot be cured. Preventing a sexually transmitted infection is easier than treating an infection once it has occurred.