www.TeachEngineering.org: Free Curriculum for K-12

Contributed by: Engineering K-Ph.D. Program, Pratt School of Engineering, Duke University

Viruses are little more than a bunch of either DNA or RNA wrapped up in a protective coat made of protein. Since they are so small they can only be seen through an electron microscope, and since they lack the machinery to do the things cells normally do, such as metabolize and reproduce, it is debatable whether they should even be considered living organisms.

To survive and reproduce, a virus must attach itself to a host cell and inject its DNA or RNA into the host (see Figure 1). The first thing the viral genetic material does is shut down the host cell's normal activity. Then the virus DNA or RNA hijacks the cell's protein-making machinery and instructs it to make new viruses instead. Soon the cell becomes so full of viruses that it bursts open, releasing them to attack host cells of their own. The whole process takes place quickly for common viruses such as the cold virus. Some viruses require less than a minute to shut down the host cell's normal activity, and in less than an hour a single virus can be replicated one hundred times.

Diagram showing how a virus infects a cell.

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Different types of viruses target different types of host cells. Cold viruses generally attack the mucosal cells lining the upper respiratory tract, while influenza viruses can also attack further down as far as the lungs. The herpes viruses that cause chicken pox and cold sores attack nerve cells in the skin, while hepatitis B is caused by a virus that attacks liver cells. Rotaviruses, which cause diarrhea, attack cells lining the gastrointestinal tract. The human immunodeficiency virus (HIV), the virus that causes AIDS, is especially deadly because it attacks some of the most important cells of the immune system, the helper T cells (also known as T4 cells or CD4 positive T cells). Because the immune system itself is attacked, the body is unable to rid itself of the virus.

Humans aren't the only ones plagued by viruses: other mammals can get rabies, for example, and monkeys and chimpanzees can get simian immunodeficiency virus (SIV), which is very similar to HIV. Flu outbreaks are not uncommon in hogs and chickens, because these domestic animals are usually kept in very close quarters. Some viruses seem not to affect insects, but the insects can carry them to humans when they bite. The viruses that cause encephalitis and yellow fever are examples. There are also more than 400 viruses that attack plants (one of the most studied is the tobacco mosaic virus), and even more viruses that attack bacteria. The latter are known as bacteriophages.

It is when the host cells burst that we start to feel sick. By this point the immune system has been alerted, and the inflammatory response that results further contributes to our malaise. If it is a cold virus that has infected us, each time we sneeze or cough we release hundreds of newly made viruses out into the environment as tiny airborne droplets. These can be inhaled directly by someone close by. Or, when we take a drink of water, we leave viruses behind on the rim of the cup. We were all taught to cover our mouths when we sneeze or cough, but in doing so our hands, if we don't use a handkerchief, are sprayed with viruses. If we pick up a pencil and use it, and then put it down, the pencil now has viruses present on its surface. Someone else can come along and use the same pencil, transferring some of the viruses to his fingers. If that someone then puts a finger in his mouth or nose, or eats a sandwich without washing his hands first, the viruses have easy access to the mucosal cells they target. Frequent hand washing is one of the most effective ways to prevent colds, and not sharing a drinking cup is another.

HIV is transmitted via either semen or blood. Since its host cells are the helper T cells, their ability to summon the B cells (see Lesson Closure section) is compromised and the production of antibodies against the virus is limited. The initial symptoms of HIV are flu-like, last a week or so, and generally occur about a month after infection. Although many helper T cells are destroyed by the virus, the body begins to respond by stepping up production of new helper T cells. For most otherwise healthy adults, this situation can be maintained for years, with the virus and the immune system at a stalemate. Eventually, though, the immune system is sufficiently weakened, the number of helper T cells declines to less than a fifth of its normal level, and the body becomes susceptible to a variety of infections. At this point the patient is said to have AIDS, as opposed to merely being HIV-positive.

AIDS is a syndrome, a collection of maladies, rather than one specific disease. Several infections tend to be characteristic in AIDS patients, and one form of pneumonia is particularly common. This form is due to a fungus that is normally found in the bodies of healthy people but is controlled by their healthy immune systems. With AIDS, though, the immune system cannot keep it under control and the lungs provide the warm, moist conditions in which the fungus can thrive. Another common AIDS ailment, particularly in men, is Kaposi's sarcoma. This is a form of skin cancer characterized by bruise-like, but generally painless, lesions. Kaposi's sarcoma rarely affects women, but women with AIDS have higher rates of cervical cancer than normal. Tuberculosis, a bacterial infection, is also common in AIDS victims, particularly in less developed nations. The chief cause of death among AIDS patients in undeveloped nations, however, is malnutrition and weight loss due the diarrhea that can originate from any of a number of bacterial, viral, or parasitic agents endemic in such areas. While these agents may be present in most of the surrounding population, like the fungus that causes pneumonia, these are normally kept under control by the immune system. Only when the number of helper T cell becomes low do these disease agents begin to thrive, cause serious illness, and eventually, lead to the patient's death.