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Viruses are intermediate between living and non-living, and at various times they have been classified as both. Most biologists hedge their bets and refer to them as "biological entities", and they are among the smallest biological entities known to exist, being visible only under an electron microscope. The virus that causes AIDS, for example, is four millionths of an inch in diameter! They are not the simplest entities, however, as prions (believed to cause Mad Cow Disease) consist merely of protein.

Viruses are often considered non-living because they cannot reproduce by themselves, they do not eat or excrete, cannot move by themselves, and because they do not grow but merely multiply, with the help of the host. When a virus infects a cell, it virtually takes over the cell and forces it to create more viruses. Without a host cell, the virus is inert.

A virus essentially consists of two parts: an outer protein casing and a particle of nucleic acid (the genome), which can be double or single stranded DNA or double or single stranded RNA. The RNA viruses tend to mutate quickly and they are more variable than DNA viruses, which makes them generally more deadly.

The virus as a whole is known as a virion, while its protein casing is called a capsid, which is made up of fragments of protein called capsomeres. Some viruses also have a lipid (fat) membrane outside the capsid, and some have a protein tail or spikes projecting from the capsid or membrane.

To illustrate the different kinds of viruses and their anatomy, let us look at a few specific viruses.

INFLUENZA virus
There are many strands of the influenza virus, and they quickly mutate and change every year. They each contain a helical core of single-stranded RNA wrapped in protein (the combination is called a nucleocapsid). The core is enclosed in a protein layer called a matrix and the outer layer is a lipid membrane.

The outer layer has numerous tiny protein spikes made of haemagglutinin (HA), and in some types of virus there are also clusters of neuraminidase (NA). Some influenza viruses are classified according to the types of HA and NA in these spikes and clusters. The H5N1 virus, for example, is the Bird Flu virus, and others include the H2N1 and H3N2). Like most influenza viruses that attack other species besides humans, Bird (Avian) Flu is a type A flu virus, which is the type responsible for flu epidemics and pandemics. Type B influenza is more localized. There is also a Type C flu virus, but it causes no

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