Viruses cause colds and the flu. Viruses are microscopic particles that invade the cells of plants, animals, fungi, and bacteria. They often destroy the cells they invade.

How do viruses reproduce? A virus first enters a cell in one of three ways: direct penetration, endocytosis, or membrane fusion. The virus takes over the cell's machinery and is thus forced to make the virus's proteins and RNA. When the virus has entered, viral RNA is released in to the cell and reverse transcriptase occurs. Reverse trancriptase is when the cell makes a DNA copy of the viral RNA. It then produces the proteins and genes the virus needs to be assemble. The virus can then be released by three ways: lytic, lysogenic, and persistent.

A virus, although not considered to be alive, does have a life cycle. First the virus attaches to a cell, which it recognizes by its surface markers. Then it penetrates the cel

l and gets inside. Next, it replicates and makes copies of itself. It then assembles itself back together and the new viruses are released.

HIV reaches the interior of a cell and the genes start to cause the cell to replicate more viruses. In this way, the infection moves from cell to cell organism to organism. It attaches to its target cell in a standard way. The GP-120 surface antigen interacts with a glyprotein cell surface maker CD4. It penetrates the cell via membrane fusion and proceeds to unwrap its RNA genes. The RNA must be turned into DNA. Using reverse transcriptase, the viral RNA is made into DNA. Thus, the viral DNA gene-copies are in the cell's genome permanently (or until it dies). The virus infection is now forever in the cell. When the cell undergoes division, the viral genes are also replicated. After the viral DNA has been made the new viruses are assembles and then released by budding.

There are three different stages of HIV. Acute is the time from when you are first infected until the antibodies are made. You have brief flu-like symptoms. Chronic (asymptomatic) is while you display the antibodies. You show signs of the disease, but you fell fine. The T4-cells start to die off and you are likely to contact other infections. Crisis is where the T4 cell counts drops below 200 per milliliter of blood. You are classified as having full-blown AIDS. This is when the opportunistic infections occur.

Some common words found in the essay are:
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Approximate Word count = 1427
Approximate Pages = 6 (250 words per page double spaced)