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HIV is transmitted from person to person via bodily fluids including blood, semen, vaginal secretions, and breast milk. Individuals who have other sexually transmitted diseases are more susceptible to the virus. HIV is different from many other viruses because it reproduces much quicker and has a high mutation rate. When HIV reproduces, different strains (types) of the virus emerge. Mutations (changes) occur almost every time a new copy of the virus is produced. Therefore, many types of HIV can be produced in a single person in one day. That is why HIV patients receive different combinations of antiretrovirals to suppress the virus. 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 have a molecule called CD4 on their surfaces, which allow the cells to detect foreign substances, including viruses that enter the body. HIV binds to the receptors on the CD4 cell wall and enters the cell. Once inside the cell, HIV begins replicating, killing the cell. Patients can transmit the virus to others during all stages of infection. 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, which kill abnormal or infected body cells, are then activated to destroy HIV–infected body cells and antibodies are produced. 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 generally do not experience symptoms during this stage. As the virus continues to weaken the immune system, patients eventually become more susceptible to infections. Patients may experience infections, such as pneumonia or tuberculosis, before the onset of AIDS.
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