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The world has now entered the second decade of dealing with acquired immune deficiency syndrome (AIDS). According to the Centers for Disease Control, the AIDS virus was first named in 1982, and the human immunodeficiency virus (HIV) was identified in 1984. It is important to make the distinction between the two acronyms, AIDS and HIV: Once infected by the HIV virus, a person may not develop the disease AIDS for years. The incubation period for developing AIDS varies from one year to 10, though experts disagree on this. The segment shows what happens in a normal immune system versus an immune system infected with HIV. Under normal conditions, disease-causing agents (pathogens) attempt to invade the body, inducing an immune response from T-cells, B-cells, and macrophages. T-cells process the foreign body so that it can be recognized by the B-cells, which in turn produce antibodies that grab the pathogens, pin them down, and mark them for destruction by the macrophages. More and more defenders descend upon the attacking virus until the invasion is neutralized. HIV acts differently than most pathogens: It seeks out the T-cells and incorporates itself into them. Then HIV either reproduces so quickly that it destroys the host cell, or it causes the genetic machinery to reproduce copies of itself, so that it can send out more virus particles to attack other T-cells. HIV doesn't always act quickly; it can hide out in the body and not reproduce immediately. But once in the body, HIV stays there forever, using the host cell as an HIV "factory." Eventually, the body's supply of T-cells becomes depleted until the immune-defense system is severely weakened and susceptible to infection by "opportunistic" pathogens, such as Pneumocystis carinii, a serious respiratory infection, and malignant growths like Kaposi's sarcoma, a vascular-type cancer. HIV is transmitted from an infected person to a healthy person in three basic ways: through sexual intercourse, through the blood system by sharing needles, and perinatally from mother to child. In the United States, the first decade of HIV infection occurred primarily among intravenous-drug abusers, people who had received blood transfusions, homosexual men, bisexual men, and all of their sexual partners. In this second decade, "heterosexual transmission will become the predominant mode of HIV transmission throughout the world," according to the World Health Organization.