Left untreated, HIV is a virus that weakens the immune systems of those it infects until they eventually develop deadly opportunistic infections. Part of a class of viruses called retroviruses, HIV kills cells known as T-cells that perform a key role in the body's immune system. A healthy, uninfected person has 800 to 1,200 of these CD4+ T cells per cubic millimeter of blood, but that number declines after a person is infected with HIV. Once the number of T-cells falls below 200, a patient becomes vulnerable to the infections and cancers associated with AIDS.

Some of the illnesses most commonly associated with a low T-cell count are pneumocystis pneumonia, the cancer Kaposi's Sarcoma, and the fungal infection thrush. According to the U.S. Centers for Disease Control and Prevention, a person has AIDS once their T-cell count falls below 200 or once they develop one of the illnesses associated with the disease.

Antiretroviral drugs keep HIV from reproducing and infecting new cells. When these drugs are used in combination, they can decrease the amount of the virus in a patient's blood and increase their T-cell count. A higher T-cell count prevents the development of complications from the disease, resulting in a healthier and longer life for those infected with the virus.

Scientists have developed three general types of these drugs and they all work to keep the virus from reproducing. When taken together these three drugs, which include protease inhibitors and other drugs such as AZT, are known as the "triple cocktail."

While the "triple cocktail" has made HIV a chronic, manageable condition for many infected with the virus, the treatment does have downsides. Even though treatment can sometimes reduce the virus' presence until it is no longer detectable in the blood, the drugs do not cure HIV. Scientists believe that the virus still resides in the patient's body, hiding in places like the lymph nodes and the brain. The drugs also have harmful side effects in some people, such as causing the onset of diabetes.

It is also crucial that patients stick to their demanding drug regimes that can sometimes require taking 25 pills a day -- some with food and others on an empty stomach. If a patient skips even one dose, the virus has a chance to make copies of itself.

Forgetting to take some of the doses can also cause the emergence of strains of HIV that are resistant to the drugs. This makes it harder for the drugs to successfully keep the amount of the virus low enough that it does not lead to full-blown AIDS.

-- By Karyn Schwartz, Online NewsHour