IStock Photo 5179977 © Jill Chen

Across the US population, the odds that a person will be diagnosed with AIDS in a year are 1 in 7,990. But this risk varies dramatically with age. For children younger than 13 the risk is 1 in 1,877,000, reflecting the fact that most persons in this group are not sexually active and are most likely to have been exposed if their mother was HIV-positive during pregnancy. The odds rise sharply once Americans become sexually active: for persons aged 20 - 24, for example, the risk is 1 in 12,650. The highest likelihood occurs between ages 40 - 44, at which the odds of being diagnosed with AIDS are 1 in 3,080. From this point the risk decreases, falling to 1 in 44,620 for a person aged 65 or older.

Why are Americans in their forties most likely to develop AIDS in a year? The main route of HIV/AIDS transmission in the United States is men having sex with other men: the odds that a male 13 or older who is diagnosed with HIV in a year was exposed this way are 1 in 1.44 (69%) compared to a 1 in 3.35 (30%) chance that a male or female of this age was exposed through heterosexual contact, and 1 in 8.53 (11.7%) odds that she or he was exposed through injection drug use. Americans now in their 40s were born in the 1960s and reached their teens in the late 1970s and early 1980s, as the gay pride movement (launched by the 1969 Stonewall riots in New York City), was growing. However, HIV was not identified until 1981 and transmission routes were not fully understood until several years later. This cohort grew up in a window when openly gay lifestyles were gaining acceptance, but before risks of HIV/AIDS and ways to avoid them were widely known.

There also are age disparities in life expectancy for persons living with AIDS. A person aged 13 - 14 has a 95% chance of surviving more than three years from diagnosis, compared to 84% for a person aged 40 - 44 and 69% for a person aged 60 - 64. Medical advances have greatly increased life expectancy for HIV/AIDS patients. One 2006 study projects that HIV-positive adults can expect to live for more than 24 years using standard antiretroviral drugs.

But the high cost of these regimes—more than $618,000 per person over that 24-year span—reinforces the importance of prevention. Ethnic and racial disparities also loom large: the estimated HIV infection rate for white Americans in 2006 was 11.5 per 100,000, compared to 29.3 per 100,000 for Hispanic Americans and 83.7 per 100,000 for black Americans. Like the HIV virus itself, HIV prevention and treatment strategies must constantly adapt to succeed.