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Bacteriophage is almost as elusive as filtrable viruses; it can scarcely be seen under a high-powered microscope, and must be cultured on a special nitrogen compound called asparagine.

She had Encephalitis Lethargica, probably caused by a filtrable virus, a type of sleeping sickness for which medicine knows no sure cure.

He was "bitten by some filtrable virus and turned into a reformer."

How do filtrable viruses get through the respiratory system to cause diseases like measles?

If organisms never seen by human eye do exist in the filtrable viruses of common colds and infantile paralysis, they might be detected by light of such short wavelength.