Virus Diseases*

Abstract

THE term ‘Virus’, formerly used in a much wider sense, has recently been given a special and restricted application. Viruses are distinguished from such long-recognised agents of infection as bacteria by their small dimensions, putting many of them beyond the limit of clear visibility by the highest powers of the microscope, and allowing them to pass through filters fine enough to retain the smallest ordinary bacteria; and by the fact that they cannot be cultivated on ordinary, non-living media. The work of Pasteur and Koch initiated an epoch of investigation on the relation of visible bacteria to different infections, so that the first clear recognition of the existence of ultra-microscopic and filterable infective agents came only in the last decade of the nineteenth century, and the present century has seen the rapid progress of investigations on their nature. The viruses first recognised—those of mosaic tobacco disease and the foot-and-mouth disease—are still far beyond the range of the most advanced microscopical methods, and, if studied by themselves, might still be regarded as living contagious principles in solution. A large number of diseases of man, animals and plants are now known to be due to infection by viruses, and methods now available have shown that the infective units of the different recognised viruses cover a wide range of dimensions.