Many of our most common diseases, measles, chickenpox, herpes febrilis and vaccinia, are caused by filtrable viruses. It is desirable that workers who are not actively investigating such disorders, as well as those who are so engaged, become familiar with this type of disease-producing agent. Therefore, through the medium of some general remarks followed by descriptions of specific work conducted in various laboratories, I shall attempt to engender a greater appreciation of the problems encountered in this field of research.
VIRUSES IN GENERAL
What are filtrable viruses?1 Innumerable answers to this question have been proposed, but not one as yet has been generally accepted. The unsettled state of affairs in this realm of research extends even to the spelling of words employed in connection with the work. For instance, both filterable and filtrable are used; virae, vira and viruses are employed for the plural of virus (there is no