The human immunodeficiency virus (HIV) is a retrovirus that causes AIDS (acquired immune deficiency syndrome). The retrovirus primarily attacks the immune defense system, making the body extremely vulnerable to opportunistic infections. Opportunistic infections occur in individuals who are immunocompromised (have weakened immune systems).
HIV is transmitted from person to person via bodily fluids, including blood, semen, vaginal discharge, and breast milk. It can be spread by sexual contact with an infected person, by sharing needles/syringes with someone who is infected, through breastfeeding, during vaginal birth or, less commonly (and rare in countries where blood is screened for HIV antibodies), through transfusions with infected blood. HIV has been found in saliva and tears in very low quantities and concentrations in some AIDS patients. However, contact with saliva, tears, or sweat has never been shown to result in HIV transmission.
Currently, there is no cure for HIV/AIDS. Patients receive antiretroviral drugs, which suppress the virus. These drugs do not reduce the risk of transmitting the disease to someone else.
HIV can infect and kill many different types of cells in the body, but the primary targets are immune cells called CD4 T-cells. The CD4 T-cells are white blood cells that help coordinate the immune system's response to infection and disease. These cells express a molecule called CD4 on their surfaces, which allows them to detect foreign substances, including viruses that enter the body. HIV binds to the receptors on CD4 cells and enters the white blood cell. Once inside the cell, HIV begins replicating.
The first stage of HIV, known as the primary or acute infection, is the most infectious stage of the disease, and it typically lasts several weeks. During this phase, the virus replicates rapidly, which leads to an abundance of the virus in the bloodstream, and a drastic decline in the number of CD4 T-cells.
The CD8 T-cells (cells that kill abnormal or infected body cells) are then activated to destroy HIV-infected body cells and antibodies are produced. An estimated 80-90% of HIV patients experience flu-like symptoms during this stage.
The next stage, called clinical latency, may last anywhere from two weeks to 20 years. During this phase, HIV is active in the lymph nodes, where large amounts of the virus become trapped. The surrounding tissues, which contain high levels of CD4 T-cells, may also become infected. The virus accumulates in infected cells and in the blood as free virus.
Patients progress to AIDS when their CD4 cell counts drop below 200 cells per microliter of blood. Healthy individuals have a CD4 cell count between 600 and 1,200 cells per microliter of blood. Individuals with a CD4 cell lower than 200 cells per microliter of blood have the greatest risk of developing opportunistic infections.
Many factors can affect how quickly HIV infection progresses to AIDS. Factors such as age, co-infections (infection other than HIV), ethnicity, geographic location, genetics, infection route (how the disease was transmitted), nutrition, pregnancy, stress, and whether or not the patient smokes or uses recreational drugs can affect the rate at which an HIV patient develops AIDS. In addition, the healthcare provider's experience treating HIV can influence how quickly the virus progresses.
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