Medicine: Wanted: A Host

Viruses are the cause of some of mankind's most deadly epidemic diseases—e.g., influenza, yellow fever, smallpox, infantile paralysis. But until a few years ago, nobody had ever seen a virus. Now, thanks to the electron microscope which makes them visible, biologists are able to study the viruses' submicroscopic world.

Viruses are the smallest known disease-producing organisms: the biggest of them is less than one-hundred-thousandth of an inch in diameter. Biologists once wondered whether a virus was a living organism or just an overgrown, active protein molecule. The dispute is still not entirely settled,...

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