In the first part of our series, we examined the importance of regular Pap smears and the increasingly common viral infection known as human papillomavirus (HPV), which is responsible for a large proportion of abnormal Pap smears. Here we describe how common genital HPV has become and how infection spreads.

Ask a random sample of American adults to name the leading sexually transmitted diseases (STDs), and HPV won't rank high on the list. But look at the statistics on which infections are the most widespread, and HPV looms large, ranked as either the first or second most common STD.

Why the disconnect? HPV is one of a number of STDs that are widespread and sometimes troublesome but generally do not cause obvious signs and symptoms. It's now known, of course, that genital HPV is the leading cause of cervical abnormalities and cancers--and is also the cause of genital warts. But few Americans associate this strangely named virus with either condition.

Who Gets It?

Most sexually active people are exposed to genital HPV at some time in their lives. An estimated 20 million people in the United States have genital HPV today, and every year, there are approximately 5.5 million new cases. HPV is one of the most common viral STDs on college campuses and seems particularly prevalent among young women in their 20s.

Because most people with HPV do not show symptoms, it can be difficult to say how, when, or from whom someone acquired HPV. Experts even disagree on whether people can eventually clear the virus from their bodies or if it is a lifelong infection. So you can see why it is difficult to estimate how many people actually have HPV.

The most dramatic projections were published in a 1997 article in the Journal of the American Medical Association (JAMA). Author Laura Koutsky, PhD, of the University of Washington in Seattle, estimated that 75% of men and women between the ages of 15 and 49 (more than 100 million people) have had genital HPV infection at some point in time. The highest rates of infection were found in adults age 18 to 28. While 100 million is an astoundingly large number of people, Koutsky characterizes HPV as a largely invisible infection, as follows:

-- 1% (1.4 million) of those infected have genital warts;

-- 4% (5 million) have an active HPV infection that causes the kind of abnormal cell changes detected by Pap smear or colposcopy;

-- 10% (14 million) have HPV infection that can only be detected by highly sensitive DNA or RNA testing;

In Koutsky's model, the remaining 60% (81 million) have only the telltale trace of a prior infection--HPV antibodies circulating in the bloodstream. A mere 25% of the sexually active U.S. population has no indication of current or prior HPV infection.

Specific Risks for Women

New data from the Centers for Disease Control and Prevention (CDC) indicate that women are at higher risk for HPV infection than men. In a study led by Katherine Stone, MD, the prevalence of HPV-16 (a high-risk type of HPV that accounts for 50% of all cervical cancer) was twice as high among women as men. Women of all races had an HPV-16 prevalence of 17.9%, compared with 8% for men. African-American women between the ages of 20 and 29 had the highest prevalence of HPV-16 at 36%.

Partner Change as Risk Factor

What about the number of partners you have? Both the CDC and the Koutsky studies indicate that risk for HPV increases with the number of sexual partners a person has had over time. Koutsky's data showed that almost 20% of people with a lifetime total of two sex partners had acquired HPV, compared with a rate of more than 80% in people with more than 10 partners.

The CDC data, which only examined risk factors relating to HPV-16, showed 7% of people with one lifetime sex partner had HPV-16. The number rose to 20.1% among those with 50 or more lifetime partners. The message of both studies: the more partners you have, the greater your chances of coming in contact with HPV and acquiring the virus.

How HPV Spreads

As the latest studies show, most sexually active people acquire genital HPV at some point in their lives. The only way you can be sure not to contract it is by abstaining from sex altogether or by practicing mutual monogamy throughout your life.

Because many of us learned about STD risks through education about HIV, we tend to think that STDs are always transmitted by bodily fluids-the major risks for HIV being blood, semen and vaginal secretions. The types of HPV that cause genital infection, however, are spread by various kinds of sexual contact, including skin-to-skin contact that may occur during foreplay or during intercourse.

It's worth noting that genital types of HPV are quite distinct from those that cause common warts on the hands or other parts of the body. Hand warts generally do not spread to the genitals, nor do genital warts spread to the hands. Also, it is unlikely for anyone to acquire warts in the mouth or throat from oral sex with a partner who has genital warts, though there is some risk.

A special note about condoms