HIV, which stands for human immunodeficiency virus, is the virus that causes AIDS. HIV destroys a certain type of blood cell (known as T-cells or CD4 cells) that help the body fight off infection. 

A person can be infected with HIV for many years before any symptoms occur, and during this time, an infected person can unknowingly pass the infection on to others. HIV can be transmitted through some forms of sexual contact, through mother-to-child transmission (MTCT) during pregnancy, delivery, or breastfeeding, and through contact with infected blood and other body fluids, such as during the use of shared injection needles, the use of contaminated skin-cutting tools, needle stick injuries in health care settings, or transfusions of infected blood.

AIDS is acquired immunodeficiency syndrome, an advanced stage of HIV infection that occurs when the immune system cannot fight off infections that the body is normally able to withstand. At this stage, the infected person becomes more susceptible to a variety of infections, known as opportunistic infections, and other conditions (e.g., cancer). Some examples include chronic cryptosporida diarrhea, cytomegalovirus eye infection, invasive cervical cancer, Kaposi’s sarcoma, lymphoma, mycobacterium avium complex, pneumocystis pneumonia, toxoplasmosis, and tuberculosis. 

At present, there is no cure for AIDS, and it is believed that most people with HIV infection will eventually die from an AIDS-related illness. However, with the advancements in HIV/AIDS therapies, including those that fight the virus itself as well as those that prevent or treat opportunistic infections, some people with HIV/AIDS—mostly in developed countries—have dramatically extended and improved lives. Unfortunately, these therapies are rarely available in resource-poor countries, where the majority of those with HIV/AIDS live. 

HIV is spread through three main modes. These modes of transmission are as a result of exposure to body fluids (blood, semen, vaginal fluids, and breast milk) of infected individuals. Specifically, HIV can be transmitted through: 

1. Sexual contact: 

2. Blood contact: 

3. Mother-to-child transmission (MTCT): 

Although any exposure through one of these methods can lead to HIV infection, not every exposure results in transmission of the infection.

How HIV is NOT transmitted?

Many myths exist about how HIV is transmitted, and many myths are culturally specific. It is important that people realize that HIV is actually quite difficult to transmit. For example, it is far less transmissible than hepatitis B or some other STIs. HIV is NOT transmitted through:

In addition, HIV is not transmitted through tears, sweat, saliva, vomit, feces, or urine. Although these substances can contain HIV, they do not contain the virus in amounts significant enough to cause infection. Extensive, continuing studies of new HIV infections over the last 20 years in many countries have not uncovered any cases of infection through these substances. To date, there is no documentation of HIV transmission through these substances. Blood, semen, vaginal secretions, and breast milk are the only body fluids through which HIV transmission has been documented. 

It is theoretically possible to transmit the virus through deep kissing if the gums have open sores or are bleeding, but this is highly unlikely. Even so, transmission in this case would be through blood rather than through saliva.

The above information is taken from EngenderHealth.