Basics of HIV/AIDS

Looking for basic information about HIV/AIDS? We’ve got answers to the most commonly asked questions. Our answers are based in scientific research, so you can trust what you read here. Looking for something not covered on this page? Visit the Health Library page.


HIV stands for “human immunodeficiency virus.” HIV was given this name because it infects humans and, over time, causes a deficiency (weakness) in the body’s immune system. The immune system helps us stay healthy by fighting germs and destroying abnormal cells that cause illness.

The term AIDS stands for “acquired immune deficiency syndrome” or “acquired immunodeficiency syndrome.” AIDS is “acquired” in the sense that people develop it as the result of an infection, specifically with HIV. The word “syndrome” refers to a group of health problems that make up a disease. In the case of AIDS, these health problems are a result of HIV-related weakening of important parts of the immune system.

You can see from these descriptions that, while HIV and AIDS are related, they are not the same thing. HIV is a virus, and AIDS is a condition caused by infection with HIV.


The immune system is the body’s protection against bacteria, viruses, and other disease-causing germs. When a person becomes infected with HIV, the virus damages and kills different types of cells, including CD4 T-cells—often called “T-cells” or “CD4s” for short. The body’s immune system needs to have both enough CD4 cells and the right balance of CD4s and other cells to fight germs and abnormal cells effectively.

In most cases, if a person’s HIV infection is not treated, then both the number of CD4 cells (their “CD4 T-cell count”) and the proportion of CD4 cells in their blood (their “CD4 percentage” or “CD4%”) will gradually drop. Without treatment, the CD4 count and CD4% will often, over a period of years, reach dangerously low levels.


If people do not get any treatment for HIV, it takes an average of about 8 to 10 years to progress from first being infected with the virus to having an AIDS diagnosis. An HIV-infected person is given an AIDS diagnosis if any one of the following three things happens:

  • Their CD4 count falls below 200—a level at which, generally speaking, a person’s immune deficiency makes them highly vulnerable to some serious illnesses.
  • Their CD4% falls below 14%; such a low CD4% is another indication of poor immune function.
  • They develop certain serious infections or cancers that have been confirmed by tests; the U.S. Centers for Disease Control and Prevention (CDC) has published a list of more than two dozen conditions that are considered “AIDS-defining.” In other words, if a person is infected with HIV and they develop one of these conditions, they are automatically given an AIDS diagnosis.


AIDS-defining conditions include many so-called opportunistic illnesses. Opportunistic illnesses, sometimes called “OIs” for short, are a group of infections and cancers that are that are uncommon in people with a healthy immune system. In some cases, these illnesses are caused by germs that our bodies are exposed to on a regular basis and that the immune system can usually fight off without difficulty.

However, these germs can take advantage of a weakened immune system to cause illness. In other words, you can think of opportunistic illnesses as germs or abnormal cells that “take the opportunity” to cause trouble when the immune system is weakened.


HIV is transmitted through four body fluids: blood, semen (and precum), vaginal fluids, and breast milk.

In order to pass HIV from one person to another, HIV-infected fluid from one person needs to get into the bloodstream of another person. Activities like sharing needles or having unprotected anal or vaginal sex are the most common ways to transmit HIV from one person to another. Using clean needles and condoms during anal and vaginal sex are VERY effective ways to prevent transmission. Recent studies have shown that other approaches can also keep people from becoming infected with the virus. Pre-exposure prophylaxis (PrEP) refers to any form of treatment that can be taken before exposure to a disease in an attempt to prevent infection. Currently, in PrEP for HIV infection, a person who is at risk for HIV infection takes a daily dose of the antiretroviral combination pill Truvada to greatly reduce their risk of becoming infected with the virus.

HIV can be passed from mother to infant before or during delivery or while breastfeeding. However, good medical care before, during, and after birth can prevent nearly all mother-to-child HIV infections.


While there is still no cure or vaccine for HIV, there are highly effective treatment options if someone is infected with HIV. These treatments are medically safe and have allowed many people living with HIV to lead longer and healthier lives.

Because HIV is a type of germ called a retrovirus, the drugs used to treat HIV are often called “antiretrovirals” or “ARVs.” These drugs work by interfering with—inhibiting—different steps in HIV’s life cycle; that is, the way HIV infects cells and then uses cells to make more HIV viruses. Most HIV-infected persons need to take three or more ARVs to gain lasting control of their HIV infection. The approach of using a combination of ARVs to treat HIV infection is often called antiretroviral therapy (ART) or combination therapy. Although ARVs may be taken one at a time, it is common for two or more ARVs to be combined in a single pill for convenience.

ARVs are often grouped into different drug classes, based mostly on the ways the drugs disrupt HIV’s life cycle and the chemicals they are made of. The main ARV drug classes are:

  • Entry inhibitors (sometimes further divided into attachment inhibitors and fusion inhibitors);
  • Nucleoside or nucleotide reverse transcriptase inhibitors (often called “nukes” for short);
  • Non-nucleoside reverse transcriptase inhibitors (often called “non-nukes” for short);
  • Integrase inhibitors; and
  • Protease Inhibitors (PIs)

If you have specific questions regarding medications, side effects, drug interactions, or other HIV-related health issues, please contact AIDS Action’s Health Library at 617.450.1432 or visit the Health Library page.