How many viruses are in your body compared to bacteria?

Dr. Leo Grady from the Laboratories for Viral Diseases/Zoonotic Diseases, Wadsworth Center, New York State Dept. of Health responds:

You have asked a good question and the answer depends very much on how you choose to define a virus. Let's begin with a conservative definition, i.e., that a virus is a very small infectious agent (20-300 nm in diameter) with a genome consisting of a single kind of nucleic acid (RNA or DNA) contained within a protein shell which is in many cases surrounded by a lipid-containing membrane, and that it can only replicate inside living cells. Using this operational definition, the answer to your question is that there are normally very few viruses found within a human being. Unlike bacteria found in the intestinal tract which provide a beneficial effect in the digestion of food, there are no known viruses whose presence is either essential or particularly beneficial to humans. If we were to test the kinds of specimens (blood, urine, stool, nasal pharyngeal secretions, and cerebrospinal fluid) from a normal, healthy person that are usually submitted for virus isolation it is unlikely that any virus would be detected. Even from persons who are ill, it is uncommon to find more than one virus by such procedures and to find more than two viruses would be very unusual. Does this mean that there are no viruses in most people during periods of good health? Not necessarily. Some viruses, such as members of the herpes group, can establish latent infections. This means that after the period of initial infection/illness is over, the virus still remains in the body, but in a quiescent form. Under such circumstances no infectious virus can be isolated. However, under certain conditions the virus can be reactivated and again produce disease. Such viruses, once present, remain with a person throughout their lives. Examples are herpes simplex and varicella zoster (varicella produces chickenpox on initial infection but remains latent and can produce a disease called "shingles"many years later). There are relatively few viruses currently known that produce such infections in humans. A third category of viral infection is chronic infection in which the patient carries and sheds virus for long periods of time, sometimes with little signs of illness (examples include hepatitis B and HIV). It is unusual for any one person to experience more than a single chronic infection. Now, if we expand our definition of a virus to include cases where some portion of a viral genome exists in a cell, but where the ability to produce an infectious virus particle has been lost (the virus has become defective in one or more ways) the answer to your question changes. This is an area of investigation that is still in its early stages, but it is highly probable that every cell in the human body contains a number of such viruses. In some cases the functioning of the remaining viral genes is likely to be beneficial and in others harmful (e.g., in underlying some cancers). One final "twist" to the answer depends on whether you want to include in your "count" of viruses in a human the bacterial viruses (bacteriophage) which in some instances may infect the bacteria in the intestinal tract. These, too, can exist in a latent and a defective form and increase the possible number of viruses in a human manifold.

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