Syphilis
By 1999, more than 12 million people around the world had gotten syphilis. More than 90% of these people (10.8 million) lived in the developing world. Syphilis got much less common after penicillin became available in the 1940s. At that time, penicillin was used as a treatment for syphilis, and often cured the disease. However, since 2000, syphilis is becoming more common again.