Sexually transmitted diseases (STDs) are generally acquired by sexual contact like AIDS, gonorrhea, syphilis, genital herpes, chlamydia etc. The causes of STDs are bacteria, parasites, yeast, and viruses. The germs that cause STDs hide in semen, blood, vaginal secretions, and sometimes saliva. STDs are serious illnesses that require treatment. Some STDs, such as HIV, cannot be cured and can be deadly. By learning more about STDs, one can learn ways to protect himself.