HIV/AIDS is the most dangerous sexually transmitted disease, with syphilis a close second, according to About.com. Part of the danger comes from the fact that there is no cure for this disease, explains Mayo Clinic.
AIDS or acquired immunodeficiency syndrome is caused by HIV, the human immunodeficiency virus, states Mayo Clinic. This disease cripples the immune system, making it impossible for the body to fight infections. People with AIDS often develop and die from infections such as pneumonia and tuberculosis because of a weakened immune system.
Syphilis is the second-most dangerous sexually transmitted disease, having infected more than 12 million people, and it is the most common STD in the world as of 2015, says About.com. Syphilis is treated with penicillin injections if caught early enough. If it is not treated, it leads to nervous system disorders, damages internal organs, causes mental disorientation and results in death.
Reports from 2013 mention an STD that is possibly more dangerous than AIDS, reports CNBC. This disease is a strain of gonorrhea that is resistant to antibiotics. This superbug can kill someone quickly, causing death from septic shock within days of infection. However, it isn't widely dispersed and has not yet caused a reported death.