Phases of human sexual response. Sexually transmitted diseases. Sexually transmitted diseases are among the most common infections in the United States, with an estimated 15 million Americans acquiring an STD every year. STDs can be arranged into three categories according to their causes.
2. Those caused by bacteria: chlamydia, gonorrhea, syphilis, and various vaginal infections, including pelvic inflammatory disease and nongonococcal urethritis.
3. Those caused by parasites: pubic lice and scabies.
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