Viruses are not cells. They do not have a nucleus or other organelles that are associated with cells. They have no metabolism and are considered genetic parasites in that they use the host cell for but one purpose---to replicate their genetic material and produce progeny virions (infectious viral particles). The effect on the host cell, which can range from no effect, to a disruption in cellular function, to killing of the host cell, is related to the disease state caused by the infecting virus.

The structural organization and morphology of viruses vary. Basically they are composed of a protein, called the capsid, that surrounds their genetic material---nucleic acid of either DNA or RNA. Together this structure is called the nucleocapsid. In addition, some viruses have an outermost covering called the envelop that is added to the virus as the virus buds from the host cell. Thus, this structure is of host cell origin. The lipid composition of this envelop is identical to the host cell membrane; it may carry some of the host cell membrane proteins as well. The virus can add its' own proteins to this envelop. An example of such an enveloped virus is HIV.

The size of animal viruses range from a diameter of approximately 20 nanometers (nm = 10^{- 9} meters) for Parvoviruses to approximately 200nm for Poxviruses. The diameter of animal cell is approximately 0.01mm (millimeter= 10 ^{- 3} meters).

The highest taxonomic grouping for viruses is the Family with subdivision into subfamilies, genera, species, and strains or serotypes.

In humans, viruses can cause a variety of diseases and have been associated with some cancers; cervical cancer, some lymphomas, and T-cell leukemias. Some viruses can be introduced into human population from other animals resulting in serious and sometimes fatal disease; the Flu virus from birds and pigs, HIV from some primates and the West Nile Virus from birds via the mosquito for example. In the case of the West Nile, the virus is responsible for the recent outbreaks of a sometimes lethal encephalitis along the east coast, from Massachusetts to Maryland; introduced into Queens New York from Israel.

An otherwise benign virus, in its' particular host species, when introduced into another species can be carcinogenic or result in a disease previously not found in the new host. This reservoir of potentially infectious and deadly pathogens represents a serious health threat in an era when every household has its' "pet"...from the traditional cat or dog to exotic "pet" primates, birds, and pigs...etc. These animals are moving into this country without the proper enforcement of health and regulatory standards representing a threat to the health of susceptible Americans.

A virus is either benign or pathogenic..none have been found, to date, to be beneficial...excepting those used as vectors in Gene Therapy.