How common are STIs? The United States has an epidemic of sexually transmitted infections (STIs). Over 70 million Americans currently have an STI.[1] 19 million new cases occur each year. Half of these are in people under 25.[2,3]

What causes STIs?
STIs can be caused by bacteria (eg, chlamydia, gonorrhea, syphilis), viruses (eg, HIV/AIDS, hepatitis, herpes, HPV), or parasites (trichomoniasis). Chlamydia is the most common bacterial STI. Human papillomavirus (HPV) infection is the most common viral STI.[1,2]

 How do you get them?
You get STIs during sexual activity. This includes vaginal sex, oral sex[4-9] and anal sex.[9-12]  A few—HPV and herpes—can even be spread by contact with infected skin. Others, such as HIV and hepatitis, can be spread through needle-sharing. You can get STIs from someone who has no symptoms.

What are the symptoms?
Most people with STIs have no symptoms. Even without symptoms, they can still pass on the infection. Some STIs cause symptoms such as an abnormal discharge from the penis or vagina, burning sensation when urinating and abdominal pain. Skin changes can also occur; these include rashes, ulcers and warts.

What are the complications?
In women, complications from infection include pelvic inflammatory disease (PID), tubal pregnancy, infertility and cervical cancer. In pregnant women, STIs can lead to miscarriage, stillbirths, preterm delivery and birth defects. In men, HPV infection can cause penile cancer.[13] Some STIs, such as HIV, can be life-threatening.

How can you find out if you have one?
Because most STIs cause no symptoms, they are not detected until complications develop. If you or your partner have had more than one sex partner or have ever injected drugs, talk to your doctor about getting tested.

 Can they be treated?
Most bacterial STIs can be treated and cured with antibiotics. Treatment does not guarantee that complications have not already occurred. A few viral STIs can be treated and occasionally cured. These are hepatitis C and B.[14,15]  However, other viral STIs can be treated for symptoms, but not cured. These include HIV and herpes. Vaccine research on viral STIs is ongoing. Vaccination for hepatitis B is effective and widely available. An HPV vaccine has recently become available and is recommended for young women.[16]  However, this vaccine does not protect against all types of HPV that cause cancer and warts. How long it works is also unknown.[16,17]

Can they be prevented?
Yes, STIs can be prevented. Avoid all sexual activity if you are single or be faithful to one uninfected partner for life. This is the only way to avoid the risk of an infection.

References:
1. Centers for Disease Control and Prevention. Tracking the Hidden Epidemics: Trends In STDs In the United States, 2000. Atlanta GA: Department of Health and Human Services, CDC; 2000. Available from: http://www.cdc.gov/std/Trends2000/default.htm. Accessed: 2006 Nov 10.
2. Weinstock H, Berman S, Cates W Jr. Sexually transmitted diseases among American youth: incidence and prevalence estimates, 2000. Perspect Sex Reprod Health; 2004;36(1):6-10.
3. Eng TR, Butler WT, eds. The Hidden Epidemic: Confronting Sexually Transmitted Diseases. Washington, D.C: National Academy Press; 1997.
4. Edwards S, Carne C. Oral sex and the transmission of non-viral STIs. Sex Transm Infect. 1998; 74(2): 95–100.
5. Hawkins DA. Oral sex and HIV transmission. Sex Transm Infect. 2001; 77(5):307–8.
6. Morris SR, Klausner JD, Buchbinder SP, et al. Prevalence and incidence of pharyngeal gonorrhea in a longitudinal sample of men who have sex with men: the EXPLORE study. Clin Infect Dis. 2006;43(10):1284-1289. Epub 2006 Oct 10.
7. Laskaris G. PL10 Oral manifestations of orogenital bacterial infections. Oral Dis. 2006;12(s1):2-3.
8. Jin F, Prestage GP, Mao L, et al. Transmission of herpes simplex virus types 1 and 2 in a prospective cohort of HIV-negative gay men: the health in men study. J Infect Dis. 2006;194(5):561-570.
9. Imrie J, Lambert N, Mercer CH, et al. Refocusing health promotion for syphilis prevention: results of a case-control study of men who have sex with men on England's south coast. Sex Transm Infect. 2006;82(1):80-83.
10. Jin F, Prestage GP, Mao L, et al. Transmission of herpes simplex virus types 1 and 2 in a prospective cohort of HIV-negative gay men: the health in men study. J Infect Dis. 2006;194(5):561-570.
11. Foxman B, Aral SO, Holmes KK. Interrelationships among douching practices, risky sexual practices, and history of self-reported sexually transmitted diseases in an urban population. Sex Transm Dis. 1998;25(2):90-99.
12. Moscicki A-B, Hills NK, Shiboski S, et al. Risk factors for abnormal anal cytology in young heterosexual women. Cancer Epidemiol Biomarkers Prev. 1999;8(2):173-178.
13. Bolan G, Ehrhardt AA, Wasserheit JN. Gender perspectives and STDs. In: Holmes KK, Sparling PF, Mardh P-A, et al. Sexually Transmitted Diseases. 3rd ed. New York: McGraw-Hill, Health Professions Division; 1999:119-121.
14. McHutchison JG. Hepatitis C advances in antiviral therapy: what is accepted treatment now? J Gastroenterol Hepatol. 2002;17(4):431-441.
15. Han SH. Natural course, therapeutic options and economic evaluation of therapies for chronic hepatitis B. Drugs. 2006;66(14):1831-1851.  16. Centers for Disease Control and Prevention (CDC). STD-prevention counseling practices and human papillomavirus opinions among clinicians with adolescent patients--United States, 2004. MMWR Morb Mortal Wkly Rep. 2006;55(41):1117-1120.
17. Mao C, Koutsky LA, Ault KA et al. Efficacy of human papillomavirus-16 vaccine to prevent cervical intraepithelial neoplasia: a randomized controlled trial. Obstet Gynecol. 2006;107(1):18-27. 

Thanks to Medical Institute for Sexual Health for their excellent information.