The human immunodeficiency virus, which is transmitted via unprotected sex and contaminated blood, weakens the immune system and eventually leads to acquired immunodeficiency syndrome. Because of the weakened immune system, the body is unable to fight off infection.
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An HIV molecule is about 1/20th the size of E. coli and 1/70th of the size of a typical white blood cell. The virus is surrounded by a membrane made of fatty materials that are dotted with small spikes made of proteins. The viral genetic material is inside the membrane. The virus also contains several proteins needed to replicate.
HIV is transmitted via contaminated body fluids, explains the Patient Education Institute, a publisher of health information. It can be contracted as a result of exposure to semen, vaginal fluids, breast milk or blood. It cannot be transmitted via sweat, saliva, tears or insects. The highest levels of HIV are usually found in the blood, though the most common route of transmission is unprotected sex. It is also commonly spread via contaminated needles.
White Blood Cells
HIV infects a white blood cell called the CD4 positive T cell, which has a protein on its surface called CD4. The protein spikes of the virus bind to the CD4 protein, which then allows HIV to bind to and enter the white blood cell. Because HIV requires the CD4 protein for infection, HIV is only able to infect CD4 positive T cells. CD4 positive T cells are needed for the immune system to fight off infections. As the virus attacks and destroys these CD4 positive T cells, the body's immune system becomes weak.
HIV has a complex life cycle, according to The Body, a physician-run website dedicated to providing information about HIV. The first step is binding to the CD4 protein, after which the virus fuses its membrane with that of the CD4 positive T cell. Once the virus has gained access to the cell's interior, it activates some of the proteins it needs for reproduction using an enzyme called protease. Next it converts its genetic material from RNA to DNA. The virus then incorporates this newly formed DNA into genes in the T cell, which causes the T cell to make more copies of the virus. Once the resources of the T cell have been exhausted, the many copies of the virus leave, via a process known as budding.
An HIV infection has many stages, according to the Patient Education Institute. In the earliest stage, also known as an acute HIV infection, the virus replicates rapidly. This can lead to mild symptoms that resemble the flu. After a few weeks the amount of virus present in the body goes down as the immune system attempts to fight off the infection. This stage can last for many years and is marked by a slow decline in CD4 positive T-cells. Eventually patients begin to develop more serious symptoms, including night sweats, chills, fever and chronic diarrhea, as the immune system becomes depleted. Finally, when the patient's CD4 positive T cells become very low, he is said to have AIDS.