|MadSci Network: Virology|
Two viruses can attack a cell at the same time. How it happens depends on what you mean by "two viruses." Are these two of the same "kind" of viruses or two different kinds? The first situation is probably fairly common, but we may not even realize that it happened. The second situation is much less likely, but it may have important public health effects. First, let's remember that there are at least three groups of viruses: those that infect animals (including people), those that infect plants, and of those that infect bacteria (often called bacteriophages). Here, we will consider only animal viruses. Viruses enter the body via the mouth (digestive tract), the nose (respiratory viruses), the genital tract (venereal viruses), or the skin (insect bites or injections). Whether an infection results depends on the outside of the virus particle and the surfaces of the cells it bumps into. Viruses have nucleic acids (RNA or DNA) inside of them, coated with protein and sometimes a fatty outer membrane. When the virus bumps into a cell, the cell's outer membrane may have complementary structures called "receptors." If these match like a lock and key, the cell engulfs the virus, the coat comes off, and the virus nucleic acid puts the cell to work making new viruses. This lock-and-key arrangement determines whether the virus can infect only people, only some other animal, or both. It also determines which cells or organs of the body the virus can infect: viruses that infect the respiratory tract may cause colds or influenza, or move on to other parts of the body and cause diseases like measles or Ebola; cells that infect the digestive tract may cause diarrhea and vomiting or move (for example) to the liver and cause hepatitis. The fit of the virus to the receptor determines whether infection happens and whether disease follows. The outside of the virus also stimulates special cells of the body to produce proteins called "antibodies" that combine with the virus to block it from causing infection. Because this antibody reaction (immunity) may take weeks, at best it helps us get over a virus disease, unless the virus was used to make a "vaccine" that can make us immune before the virus reaches us the first time.
One infected cell may produce hundreds of new viruses. By the time the disease occurs, many thousands of cells may be infected and producing new viruses. These new viruses are released from the cell, often in bunches, and are almost identical. Sometimes the host cell makes mistakes in copying the virus nucleic acid, causing some genetic variation. However, the outsides of these new viruses tend to be pretty much all the same, and they will usually infect only some more of the same kind of cells that produce them ¯ either in the same host animal or, if they can get out, other animals. Viruses produced in the respiratory tract are usually shed in droplets from the mouth and nose; viruses produced in the digestive tract are usually shed in feces but sometimes also in vomitus. One person or animal is usually infected with only one kind of virus at a time, so the bunches of virus they produce and shed are all the same. Then, if this virus infects another person, even if two of the same kind of virus bump into a single susceptible cell, the infection will still be all of one type. However (much less likely), if viruses from two different infections get mixed up in the air or in sewage and get into a single person, it is possible that two different viruses will attach to receptors on the same cell and infect that cell together. If the two viruses are closely related, their nucleic acids may "recombine" while they are being copied. When this happens, part of the genetic information from one virus is linked to the genetic information from another virus, and the new virus that is produced is a "hybrid." If you read about influenza that is traveling about in populations you will see that the outside of the influenza virus has an H type and an N type. For example, you might read that this year's main influenza virus is type H5N2. A reason to get a new flu vaccination each year is that the vaccine has to be made to immunize us against this type. Somewhere out in the world, a double infection as described above may happen between two different types of influenza viruses and some of the new virus may be type H5N4. Once this new type is established, a new vaccine is needed, or people won't be protected. Development and production of vaccines are expensive, and distributing them and getting people to use them is difficult. So, here is at least one situation where your question about two viruses infecting one cell may have serious public health results. Of course, influenza viruses that develop in birds, and sometimes other animals, may also be transmitted to people. Not many other kinds of viruses can jump host species like this, but it is possible that Ebola was transmitted to people from apes, perhaps from eating "bush meat." Viruses and other infectious agents that are transmitted from animals to people are said to be "zoonotic"; once this happens, the virus can usually be transmitted from person to person after that. Obviously, this too is an important public health concern.
Vogel, G. 2004. Epidemiology. Ebola outbreaks may have had independent sources. Science 303:298-9.
Webster, R. G. 1999. Antigenic variation in influenza viruses, p. 377–390. In E. Domingo, R. G. Webster, and J. J. Holland (ed.), Origin and evolution of viruses. Academic Press, San Diego.
Dean O. Cliver
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