One of the most sexually transmitted diseases, Gonorrhea, is becoming resistant to antibiotics.   With over 600,000 cases a year, the bacteria that causes Gonorrhea is mutating and the antibiotics are starting to fail.  This comes from the CDC, which is urging people to practice safe sex.

Especially now as the population grows and more antibiotics are being used, it's just a matter of time before other diseases become drug resistant.  Read more about it from yahoo and the Scientific American