Early workers in microbiology used the term virus in a broad sense to indicate any specific agent of infectious disease. In recent years, however, use of the term has been limited to a group of agents lying near or below the limits of resolution of the ordinary compound microscope which are usually able to pass filters retaining ordinary bacteria and which can be cultivated apart from the animal or plant body only in the presence of living cells.
In this restricted sense the viruses, like bacteria, fungi, protozoa, spirochetes and Rickettsia, form a group of disease agents having fundamental properties in common. That no group has greater economic or hygienic importance is apparent from the fact that the inciting causes of such diseases as poliomyelitis, the common cold, influenza, trachoma and yellow fever are viruses. Knowledge concerning them has increased rapidly in the last two decades, with the result that
THYGESON P. VIRUSES AND VIRUS DISEASES OF EYE: I. PROPERTIES AND NATURE OF VIRUSES. Arch Ophthalmol. 1943;29(2):285–300. doi:10.1001/archopht.1943.00880140131010
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