The paper is a concise review of recent work on diseases classed as filterable virus infections. Of these, 3 groups are prominent : the smallpox-like diseases, those caused by Rickett-sia, and those, such as poliomyelitis and rabies, affecting especially the nervous system. Attention is called to the fact that filterability is not a good basis for classification and further to the fact that in almost all cases little or nothing is known of the life history of the virus. In some cases this is due to the fact that no animals other than human beings are susceptible. A list of recent publications is appended.