4.1 CLASSIFICATION ON THE BASIS OF DISEASE

 

The first, and most common, experience of viruses is as agents of disease and it is possible to group viruses according to the nature of the disease with which they are associated. Thus, one Chapter 4 Outline 4.1 Classification on the basis of disease 4.2 Classification on the basis of host organism 4.3 Classification on the basis of virus particle morphology 4.4 Classification on the basis of viral nucleic acids 4.5 Classification on the basis of taxonomy 4.6 Satellites, viroids, and prions 50 PART I WHAT IS A VIRUS? can discuss hepatitis viruses or viruses causing the common cold. This is attractively simple. However, this method of grouping viruses, though reflecting an important characteristic, suffers from serious deficiencies. First, this approach is very anthropomorphic, focusing as it does on diseases that we recognize because they affect humans or our domestic livestock. This ignores the fact that most viruses either do not cause disease or cause a disease that we do not recognize because of a lack of understanding of the host; for example we understand little of the diseases caused by viruses of fish or amphibians. Similarly, it is possible for a single virus to cause more than one type of disease; a good example of this is varicella zoster virus which causes chickenpox in a first infection but when reactivated later in life causes shingles. This problem is compounded when considering viruses which infect more than one host as it is common to find that it causes either no disease in one host while dramatically affecting the other or that it may cause different diseases in different hosts. A classification based on disease, while it may be helpful in some settings, also fails in the important feature of being able to use the groupings to predict common fundamental features of the viruses in question. In the case of agents of hepatitis and the common cold, many different viruses with very different molecular makeups are involved and studying just one of these tells us little, if anything, about the others.

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