AIDS

Although the public—and most doctors—first learned about AIDS in the 1980s, the disease has been around since at least the 1950s and probably much longer than that. Left untreated, it eventually kills most people who have it. Fortunately, a combination of medications is now available to control the condition—although it still cannot be cured.

From a biological point of view, AIDS is caused by a virus (the Human Immunodeficiency Virus, or HIV) that progressively cripples the immune system. Usually the most severe symptoms—which include significant weight loss and diarrhea—don't appear for many years after the initial infection, which is part of the reason why physicians didn't learn about AIDS sooner. As the immune system fails, other germs and viruses, including some that don't usually make people sick, become life-threatening. These so-called opportunistic infections include various kinds of pneumonia, fungal and yeast infections and even some cancers.

But biology alone can't explain why the disease became so widespread. (The World Health Organization estimated in 2006 that at least 40 million people were HIV-positive, most of them in sub-Saharan Africa.) Rapid urbanization and social upheaval played key roles in first spreading the illness through central Africa. Poverty and lack of access to basic healthcare ensured that HIV remained a problem in the developing world even after treatments were discovered. And the stigma often associated with the disease kept most governments and societies—whether rich or poor—from acting more quickly and vigorously to halt its spread.

How Do People Become Infected?

HIV is most commonly transmitted through sexual intercourse in just about any combination you can think of. Oral sex, vaginal sex, anal sex, sex between men and sex between men and women have all been proven to spread the infection. The virus is also transmitted through the use of contaminated needles, tainted blood transfusions or organ transplants, or from an infected mother to her uninfected child during labor or breast-feeding.

Using condoms during sexual intercourse is highly effective in preventing the transmission of HIV. Needle-exchange programs in Canada and the United Kingdom have dramatically decreased infection rates among intravenous drug users. The blood supply and organ donations in most developed countries are now carefully screened against HIV. Giving women anti-AIDS medication just before they give birth significantly cuts the chances that they will pass on the infection to their newborn.

Who Gets HIV?

Although AIDS was first identified in the U.S. among homosexual men, the disease is more common worldwide among heterosexual men and women.

Women and girls are particularly vulnerable to HIV for biological and sociological reasons. In fact, in some parts of sub-Saharan Africa, married women are at greater risk of developing HIV than single women. Several studies have also shown that male circumcision decreases—but does not eliminate—a man's chances of passing on the virus during vaginal intercourse.

Health experts in the U.S. are increasingly concerned at the way that HIV, for reasons that are only partly understood, has taken hold in the African American community. Studies now show that young, sexually active black Americans are more likely than white Americans to become infected with HIV even when their partners do not come from high-risk behavior groups.

What Does HIV Do to the Body?

HIV doesn't destroy the immune system right away or all at once. A lot depends on a person's previous medical background and genetic makeup. Indeed, some people become severely ill very quickly after an initial HIV infection while others may live in apparent good health for years even without medical treatment.

Tuberculosis is a common cause of death among AIDS patients in poor countries. Malnourishment and malaria also exacerbate the condition. Even with anti-viral treatment, men and women who are HIV-positive are more prone to pneumonia and other infections.

What About a Vaccine?

Curing HIV has so far proven impossible. Designing an effective vaccine that is also safe is daunting. One potential vaccine that was based on protein bits from HIV's outer coating, which looked promising in the laboratory, failed in human trials in 2003. More recently researchers have started combining one or more vaccine candidates to see if they can work better together. Progress remains slow. Meanwhile, one of the worst scourges in human history continues its tragic devastation.

—By Christine Gorman