|MadSci Network: Virology|
Dear Maribel: Viruses must be very successful because they seem to be everywhere. There are kinds of viruses that infect people and other animals, viruses that infect plants, and viruses that infect bacteria (which they can do because viruses are much smaller than bacteria). Animals, plants, or bacteria that are infected with viruses are called “hosts.” Being infected with a virus almost never does the host any good, but some viruses cause mild infections, whereas others are deadly. I will concentrate on the kinds of viruses that infect people and other animals. Viruses don’t have any way to swim, fly, or walk. They ride along with fluids, like water or the droplets of a cough or sneeze. Our bodies are made up of cells, which have a membrane on their outsides. Viruses attach to places on cell membranes that are called “receptors.” Most viruses are very choosy about the receptors they will attach to — in many cases, viruses that infect animals won’t infect people, and vice versa. Furthermore, viruses that cause colds or influenza seldom infect our intestines, and viruses that cause vomiting and diarrhea seldom infect our sinuses or lungs. This is largely controlled by which cells have receptors for the virus. Viruses that cause colds and influenza are shed as droplets in sneezes and coughs, whereas viruses that cause diarrhea are shed in feces. The droplets travel through the air to other people and cause new infections; the viruses in feces travel from person to person in contaminated water or food, or on unwashed hands. A very few viruses are transmitted by sexual contact (for example, the AIDS virus), or even by kissing (some herpesviruses). When a virus is inhaled or swallowed, it rides along with whatever it is in until it contacts the receptor on a cell that it likes. When the virus contacts the right receptor, the cell “engulfs” the virus particle (sort of slurps it in). The virus loses its outside coat when it gets into the cell, and the genetic material (RNA or DNA, depending on the type of virus) inside the virus gets out into the cell. This genetic material takes over the cell, making the cell stop whatever it is doing and make virus. One part of the cell makes more virus RNA or DNA, and another part makes the coat of the virus. These parts get put together to make new virus, almost like an assembly line in a factory. An infected cell may make hundreds or thousands of new viruses, which then either leak out of the cell or burst it. The new virus particles infect other cells in the host, while some get out to “look for” another host. If enough cells get infected and stop what they ought to be doing for the host, the host gets sick. The kind of sickness depends on which cells the virus has killed. Viruses only multiply in infected cells, not in the environment. People (and other hosts) have their own defenses. Some defenses work for short periods against almost all viruses, and others can protect us against ever being infected with the same virus again. Long-term protection comes from proteins our bodies make that are called “antibodies.” Kids get vaccinated against a lot of diseases that used to kill many children; many of these diseases were caused by viruses. If we have been vaccinated against measles, mumps, and other such diseases, the viruses that cause them, if they get into our bodies, are stopped by the antibodies that are waiting to protect us. Another way to prevent virus infections is by sanitation — cover your mouth when you cough or sneeze, wash your hands thoroughly after using the bathroom and before eating. There is one more kind of viruses that I should mention: those that are transmitted by bites. Some come from mosquito bites, after the mosquito has bitten an infected person or other animal. We are reading a lot about West Nile virus, which is transmitted this way. Another, very different virus that is transmitted by bites is rabies. Infected dogs or other animals (bats, skunks, raccoons, etc) sometimes bite people and transmit the virus infection. In this case, it is important that the person who was bitten get vaccinated quickly; otherwise, the rabies virus infection is likely to lead to a very terrible death. Fortunately, not very many virus diseases are transmitted from animals to people or from people to animals, but influenza sometimes goes this route. Viruses that infect plants or bacteria don’t seem to infect people. Virus particles are too small even to see with a microscope that lets us look at bacteria. Still, viruses are powerful and can cause serious illnesses or death. There aren’t enough vaccines to protect us against all of the kinds of viruses that threaten us, so we have to protect ourselves as much as we can by sanitation. Remember to wash your hands! Dean O. Cliver
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