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Infectious Diseases

Author(s): Christine Herrmann, PhD

Example of an Infectious Disease - AIDS

AIDS, caused by the virus, HIV, has become the most devastating infectious diseases of our lifetime. Since 1983, when HIV was first identified, approximately one million Americans have been infected. In 2006, approximately 40 million people were living with HIV infection worldwide, over four million became newly infected, and about three million died of AIDS. The epidemic is growing most rapidly among minority populations and women.

HIV infects white blood cells, primarily those called T cells, which are part of our immune systems. HIV destroys the ability of these cells to fight infection by other agents. Therefore, HIV-infected patients sometimes die from infection by other viruses, bacteria, or other agents that are not normally harmful to healthy individuals. Some AIDS patients also develop unusual cancers because of defects in their immune systems. HIV is not easily spread from person to person because it requires direct contact with the body fluids (blood and semen) of an HIV-infected person and it does not survive for very long in the environment. Transmission can occur through sexual contact, sharing of contaminated needles, transfusion of infected blood or blood products, and from mother to infant. There has not been documentation of HIV transmission in saliva or by blood carried by biting insects such as mosquitoes. 

There usually is a long lag period from the time someone is infected with HIV until the person begins to experience the symptoms of AIDS. (HIV infection does not follow the typical disease progression pattern described earlier, and the length of time to progression to AIDS can be highly variable.) Soon after infection, an individual may experience flu-like symptoms, but then remain asymptomatic (without symptoms) for up to a decade. However, during this phase, the virus continues to replicate and the infected person is contagious.

The steady increase in the number of HIV particles during the period while the virus is reproducing leads to a gradual decline in the level of immune system cells, known as CD4-positive (CD4+) T cells. A normal person has about 1,000 CD4+ T cells in a milliliter of blood. Once CD4+ T cell numbers fall to 200 cells per milliliter, the patient enters the phase of HIV infection known as AIDS. From this point on, it becomes increasingly difficult for patients to fight off infections. Signs and symptoms of HIV/AIDS include tiredness, fever, loss of weight, diarrhea, and swollen glands.

As of yet, there is no effective vaccine to prevent AIDS, and there is no cure. There are drugs that will reduce the number of HIV virus particles in the patient’s body (the viral load), which improves the length and quality of life. However, these drugs do not rid a person of HIV. Moreover, the drugs may become ineffective over time as the virus mutates and becomes resistant to the drugs. Once treatment is ceased, virus levels go back to earlier levels.

Currently, there are four classes of anti-HIV drugs that block three essential steps in the virus reproductive cycle: the entry phase (where the virus particle fuses with the host cell); the reverse transcription step (where the virus makes a DNA copy of its RNA genome); and the protease step (where a virus protein chops long strands of virus proteins into smaller, functional units). These usually are given in combination, as a “cocktail” called “highly active antiviral therapy,” or HAART for short. HAART is very effective for many people, at least for a period of time, but the treatment can produce unpleasant side effects and is too expensive for most people in the developing world. Most of all, it is important to remember that HAART is not a cure for AIDS.