HIV destroys CD4 positive (CD4 +) T cells, which are white blood cells crucial to maintaining the function of the human immune system.

As the HIV infection progresses, there is ongoing damage to immune defense cells and the body becomes increasingly less able to fight off infection. Individuals with advanced HIV disease become susceptible to opportunistic infections that don’t show up in people with healthy immune systems.

Not everyone who has HIV infection develops AIDS. Most people who are infected with HIV can carry the virus for years before developing any serious symptoms. But over time, HIV levels increase in the blood while the number of CD4 + T cells decline.

Experts estimate that about half the people with HIV will develop AIDS within 10 years after becoming infected. This time varies greatly from person to person, however, and depends on many factors, including a person’s health status and health-related behaviour.


People are said to have AIDS when they have certain signs or symptoms specified in guidelines formulated by the U.S. Centers for Disease Control and Prevention (CDC).

The definition of AIDS given by CDC’s includes:

1. HIV-infected people with fewer than 200 CD4 + T cells per cubic millimeter of blood (compared with CD4 + T cell counts of about 1,000 for healthy people)

2. People with HIV infection have at least one of more than two dozen AIDS- associated conditions, due to HIV’s attack on the immune system


AIDS-associated conditions include opportunistic infections by bacteria, fungi, and viruses.

Opportunistic infections are caused by germs that are around us all the time but normally they can be fought off by a healthy immune system. Once the immune system is sufficiently weakened, such infections will develop and produce a wide range of symptoms.

Some of these symptoms can be very severe. When the immune system is weakened certain cancers also become more common.