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Viruses: One of Human's Strongest Enemies

Author: Jason Lavender, Duke University
Editor: Wei-Chung Chen

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What are viruses? Viruses are responsible for causing the person to become sick due to a cold or flu. On a larger scale, viruses are also responsible for causing many deadly diseases and this include AIDS (acquired immunodeficiency syndrome), Ebola hemorrhagic fever, infectious hepatitis and herpes.

Compared to bacteria and normal cells, virus particles are nothing alike. Virus particles are about one-millionth of an inch (17 to 300 nanometers) long. Viruses are about a thousand times smaller than bacteria, and bacteria are much smaller than most human cells. Viruses are so small that most cannot be seen with a light microscope, but must be observed with an electron microscope. Viruses are made of three major components. First components are nucleic acids, which include DNA and RNA and they provide a set of genetic instructions for future viral reproduction. Secondly, there are protein coatings that protect the nucleic acids. Lastly, there are lipid coatings that surround the protein coatings, but this is not present in all viruses. Viruses with lipid coatings are called enveloped viruses as opposed to naked viruses. Shape of the virus varies greatly ranging from round popcorn balls to a spider or the Apollo lunar lander.

One thing that greatly distinguishes viruses from bacteria and cells is that they do not carry enzymes needed to carry out the chemical reactions for life. Instead, they carry only one or two enzymes that decode their genetic instructions. Thus, viruses depend on host cells for viral production and nutrient. In this case, host cells include bacteria, plant cells, and animal cells. Outside of a host cell, viruses cannot function. For this reason, viruses are sandwiched in between living things from nonliving things. Most scientists agree that viruses are alive because of what happens when they infect a host cell. Viruses are everywhere around people, and waiting for the opportunity to infect host cells that would provide the necessary nutrients for survival. Since they possess small size, they can enter into our body through the nose, mouth or breaks in the skin. Once they enter inside the body, they would find a particular host cell to infect. Specifically, cold viruses will attack cells that line the respiratory tract or digestive tract. In addition, HIV (human immunodeficiency virus), which causes AIDS, targets white blood cells such as T-cells that protect the body from foreign particles.

After they enter into the body, viruses follow a particular cycle for the reproduction of new viruses. This cycle is called the lytic cycle. First, the virus particle attaches itself to the host cell. Second, the particle releases its DNA or RNA into the host cell. The injected genetic instruction recruits the necessary enzymes inside host cell. These enzymes will be responsible for making different components of a complete virus. After these parts are completed, they are assembled together to form new virus particles. Finally, these new viral particles would break out of the host cells and invade other host cells. There are two ways that the viruses break out of the host cell. First, they simply kill the host cell by breaking open the host cell. The second way is by pinching out from the cell membrane and break away (budding) with a piece of the cell membrane surrounding them. This is how enveloped viruses leave the cell. In this way, the host cell is not destroyed.

During the viral attack, the human body responds to the infection by producing chemicals called pyrogens that cause the body temperature to rise. This rise in temperature helps to fight the infection by slowing down the rate of viral reproduction. The rationale is that chemical reactions in the human body operate at an optimal temperature of 98.6 degrees Fahrenheit (37 degrees Celsius). If your temperature rises slightly above this, the reactions slow down. This immune response continues until the viruses are eliminated from your body.

While some viruses go through the lytic cycle for generating new viruses, there are viruses that undergo a different type of cycle. This cycle is called the lysogenic cycle. In this cycle, some viruses, such as herpes and HIV, do not reproduce right away. Instead, they mix their genetic instructions into the host cell's genetic instructions. When the host cell reproduces, the viral genetic instructions get copied into the host cell's offspring. The host cells may undergo many rounds of reproduction, and then some environmental or predetermined genetic signal will stir the "sleeping" viral instructions. The viral genetic instructions will then take over the host's machinery and make new viruses.

To reduce the spread of viruses, there are several precautions that people can take. First, people can cover their nose or mouth when they sneeze. Another methods include washing your hands after you leave the restroom. Lastly, it is important to avoid contact with the bodily fluids of others.

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