For many reasons it is difficult to determine the exact prevalence (number of currently infected people) or incidence (number of new cases per year) of sexually transmitted infections.

However, the STI incidence rate was estimated at 18.9 million per year for the year 2000¹- a 6 million increase over a 1996 estimate.² Below are incidence and prevalence rate estimates for STIs:^2,3

STIs in the US
Incidence and Prevalence Estimates
STI	Incidence	Prevalence
HPV	5.5 million	20 million
Trichmoniasis	5 million	unknown
Chalmydia	3 million	2 million
Genital Herpes	1 million	45 million
Gonorrhea	650,000	unknown
Hepatitis B	120,000	417,000
Syphilis	70,000	Unknown
HIV	56,300	1.1 million

Reviewed by: Jennifer A. Shuford, MD, MPH
Date: October 2008

References:
1. Weinstock H, 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.
2. 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, Centers for Disease Control and Prevention; 2000. Available from: http://www.cdc.gov/std/Trends2000/default.htm. Accessed: 2008 Sept 10.
3. Centers for Disease Control and Prevention. HIV/AIDS in the United States Fact Sheet. Atlanta, GA: Department of Health and Human Services, Centers for Disease Control and Prevention; 2008 Aug. Available at: http://www.cdc.gov/hiv/resources/factsheets/us.htm#1. Accessed 2008 Sept 10.