Craig C. Freudenrich
When you have a cold or flu, your body is being invaded by a virus. Viruses are small packages of nucleic acid (DNA or RNA) surrounded by protein coatings. Some viruses also have lipid envelopes surrounding their protein shells. Viruses are much smaller than cells (0.04 to 0.07 millimeters compared with 1 to 10 millimeters for cells). Viruses are not considered organisms because they cannot reproduce or metabolize on their own. Instead, a virus requires a host cell to replicate, or to produce copies of itself. These host cells come from your cells and viruses infect the host cells in the following manner:
1. You pick up a virus in many ways. You may touch an infected surface or breathe in an airborne virus. Some viruses are transmitted sexually and through body fluids.
2. The virus attaches to a host cell (e.g. nasal epithelium, intestinal epithelium). The protein coat or lipid envelope allow it to attach.
3. It releases its nucleic acid into the host cell. This can happen in two ways. The virus can inject the nucleic acid into the host cell or the host cell can take the virus inside, where the virus breaks apart and releases its nucleic acid.
4. The virus's nucleic acid recruits the host cell's enzymes. It directs the enzymes to make parts (nucleic acid, proteins) for new viruses.
5. The parts are assembled into new viruses within the host cell.
6. The new viruses break free from the host cells in two ways. The newly formed viruses can break the host cells open and destroy them, a process called lysis. Alternatively, new viruses can bud off from the host cell, thereby preserving it. How the new viruses are released depends upon the type of virus.
In some types of viruses (such as herpes and the human immunodeficiency virus, or HIV), the viral DNA can become part of the host cell's DNA but does not cause new viruses to be made. Instead, the viral DNA reproduces along with the host cell's DNA. The viral DNA waits for some environmental trigger. Once triggered, the viral DNA rapidly takes over the host cell to produce more new viruses. This is why a person infected with HIV may not develop AIDS for years after the initial exposure to the virus. Finally, some viruses can last a long time in the environment before finding a proper host cell to infect.
A virus is a very tiny collection of DNA or RNA genes enclosed within a protein envelope. That genetic material is useless, however, unless the virus has a host cell. Viruses can't reproduce without invading a living cell, but once they find a cell, they replicate and reproduce, spawning countless copies of their genetic material. Eventually, this reproduction will kill the host cell, and the virus will take over adjacent cells.
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