In its restricted sense the term "virus" is applied to agents of disease which so far as known do not multiply in any medium other than living cells of a susceptible host and which pass through standard filters holding back bacteria. Burnet1 has reviewed the speculations concerning the origin of viruses and their relations to other forms of life. One hypothesis assumes that viruses may be fragments from cells of high forms, footloose genes which have found ways of surviving by passage from host to host; definite evidence of any such process has not been advanced. Another possibility is that viruses are descendants of precellular forms of life which failed to survive except through the adoption of a strictly parasitic, intracellular existence. Filtrable saprophytes have been described the relationship of which to virus awaits investigation.
The dominant view of the origin of viruses is that they are degraded descendants
THE ORIGIN OF VIRUS. JAMA. 1945;129(6):449. doi:10.1001/jama.1945.02860400033012
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