The term STD, then, is more descriptive of the means of transmission rather than of etiology. Of over 20 diseases which are most commonly transmitted by sexual contact, the top five in order of prevalence are Chlamydia, gonorrhea, human papillomavirus, genital herpes, and syphilis. Chlamydia, caused by the bacterium Chlamydia trachomatis, can be easily cured once diagnosed. The gonococcus bacterium that causes gonorrhea multiplies quickly in moist, warm areas such as the cervix, urethra, mouth, or rectum. Gonorrhea is highly contagious, the early symptoms are usually mild, and some individuals never develop symptoms.
Untreated gonorrhea can spread to the bloodstream and infect the heart, the joints, or the brain.
As with other viruses, HPV is impossible to eradicate. Once an individual is infected with HSV, they remain that way for life. Sores and other symptoms do not have to be apparent for genital herpes infection to be passed from one individual to another. Its primary stage is characterized by open sores at the site of infection. In spite of old superstitions, syphilis cannot be contracted by contact with toilet seats and door knobs.