Medicine: The Ultimate Parasite

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"If You're a Tulip." Thanks largely to chemists like Stanley (who now runs the University of California's Virus Laboratory) and the electron microscopists, a virus can now be defined as an infectious particle that has no metabolism of its own and reproduces itself only by taking over the metabolic processes of the living cell it invades. Viruses are the ultimate parasites. They parasitize everything in nature from bacteria and flowering plants up through invertebrates such as mosquitoes, and the vertebrates from fish, amphibians, reptiles, birds and mammals to man.

There is no such thing as a beneficial virus, though some do no harm. Whenever a virus has a detectable effect, it is bad. All healthy tulips are solid-colored. The tulip-streak (or "breaking") virus creates variegated color patterns of great beauty in the eye of the human beholder (and of great cash value to Dutch growers). But, says Stanley, "if you have the tulip-streak virus and you happen to be a tulip, you're sick.'' This lack of evident purpose in viruses leaves teleological philosophers at a loss. Yet viruses must have influenced evolution through natural selection. In deed, the close resemblance between the virus' core of nucleic acid and the gene, or "unit of heredity;" suggests that virus particles are lost genes in search of evolution.

No man knows how many viruses there are or how to classify them. John Enders and six other internationally famous virologists* have just made a stab at classification in Virology, conceding that they are making "some dogmatic statements and sweeping suggestions based on grossly inadequate knowledge." They recognize 400 viruses as infecting vertebrates, rate 50 of them, including rabies, as unclassifiable, and put the rest in six groups (see chart). They leave out the large "mantle viruses'' of parrot fever and trachoma, which are vulnerable to antibiotics and other drugs.

A Closer Look. Thus viruses got defined and classified. But just how the virus core gets into a cell remained a mystery, even after Dr. Robley C. Williams, a member of Stanley's California team, devised the method of plating the particles with gold or uranium to get clearer electron micrographs. Then, two years ago at Cambridge University's Cavendish Laboratory, Drs. Sydney Brenner and Robert W. Home made an illuminating refinement on electron micrography, revealing far more intimate details of virus structures and differences, and clues to how viruses work.