The Biological Nature of the Viruses^*

Abstract

THE viruses are a group of agents, the existence of which would certainly be unknown to us but for the changes produced by their presence in the bodies of higher animals and plants. They seem to have one property at least of living organisms, in being capable, under appropriate conditions, of indefinite reproduction. We know nothing of their intrinsic metabolism: it has even been asserted that they have none. Few of them have yet been rendered visible by the microscope; it is, indeed, a question for our discussion whether any of them have yet been seen or photographed. It is a question, again, whether any of them, or all of them, consist of organised living units, cells of a size near to or beyond the lowest limits of microscopic visibility; or whether, as some hold, they are unorganised toxic or infective principles, which we can regard as living in a sense analogous to that in which we speak of a living enzyme, with the important addition that they can multiply themselves indefinitely. Some, however, would attribute this, not to actual self-multiplication, but to a coercion of the infected cells to reproduce the very agent of their own infection.