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In 1996 the number of Americans with a new STI/year was estimated to be about 12 million. This number was recently revised to 18.9 million in 2000,1 a significant increase.

Adolescents and young adults are disproportionately affected. Nearly half (48%) of new STI cases occur among 15- to 24-year-olds each year, although they comprise only one fourth of the sexually active population.1 

Human papilloma virus (HPV) is the most commonly acquired STI, followed by trichomoniaisis & chlamydia. These three account for 88% of all new adolescent/young adult cases (15-24).1 STI Estimate – 

Although it is difficult to know the exact number of new cases of various STIs each year (called incidence) or the total number of individuals with existing infections in the population (called prevalence), some estimates based on various studies are shown here.

Note that HPV is the most common STI in terms of new cases each year. HPV is the primary cause of cervical cancer in women. Genital herpes (another viral infection), however, has the largest number of people infected in America—an astounding 45 million or more.
STI                  Incidence *                    Prevalence **
HPV                 5.5 million                           20 million
Trichomoniasis    5million                             unknown
Chlamydia          3 million                            2 million
Genital herpes     1 million                         45 million
Gonorrhea        650,000                             unknown
Syphilis             70,000                            unknown
Hepatitis B       120,000                              417,000
* Number of new infections occurring in the population in one year
**  Number of infections found in the population including those from previous years
1. Centers for Disease Control and Prevention. Tracking the hidden epidemics 2000: Trends in STDs in the United States. [Accessed 1May04; 22Jul04, Medical Inst Advisory,]