The poison produced in the course of an infective disease and capable of reproducing the disease when inoculated into another person is called a virus. It may contain visible microorganisms, but in some instances no organisms have been demonstrated, though they are presumed to exist.
When the infective virus can pass through a porcelain filter it is called filterable virus. A virus which has had its potency diminished by chemical or other means is called an attenuated virus.