HIV — the human immunodeficiency virus — destroys T helper cells in the blood, which compromises the immune system. As an infected person’s immune system gradually weakens, it becomes harder and harder to fight off infections such as tuberculosis. Without treatment, the person eventually dies from opportunistic infections.

Since 1981 when AIDS was identified, science has failed to find a cure or vaccine for the disease, and none is anticipated in the near future. Some 35 million people around the world have died of AIDS, and roughly 34 million carry the virus today, according to UNAIDS estimates.

A regimen of drugs, known as antiretroviral therapy, has sustained life for millions of HIV-positive people in recent years and has become less expensive over time. But ART has become harder to find in some developing countries as the global recession has reduced the ability of governments and foundations to fund treatment programs, despite lower costs of drugs.

Effective behavioral and preventive interventions, including male circumcision, condom use and measures to stop mother-to-child transmission, remain very important, but clearly are not enough to stop the spread of HIV.