360 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



all virus diseases are followed by a lasting immunity in recovered 

 hosts, the fact that many virus-infected cells contain inclusion bodies, 

 and the fact that, as a group, viruses are smaller than ordinary bac- 

 teria. It should be emphasized that no single one of these properties 

 may be used to differentiate viruses from bacteria and that, despite the 

 attempted separation based on the properties just mentioned, viruses 

 have nevertheless been generally considered as merely small ordinary 

 living organisms, somewhat similar to the bacteria. Needless to say, 

 too ready acceptance of this conception delayed the chemical approach 

 to the problem for several years. 



The facts that viruses may multiply or reproduce, that they may 

 change or mutate and adapt themselves to new conditions, that they are 

 specific in their action in that a given virus occurs or causes disease 

 only in certain hosts, and that a lasting immunity follows most virus 

 diseases have been used as arguments for the living nature of viruses, 

 for these properties have been generally regarded as characteristic of 

 living things. For a time there were but few dissenters, and the large 

 majority of the workers in the virus field saw no reason why viruses 

 should not be considered small, invisible, living organisms. This con- 

 viction became even stronger with the discovery that some viruses 

 were actually larger than certain bacteria. However, in 1931 

 Galloway and Elford reported that the virus of the foot-and-mouth 

 disease of cattle was only about 8 to 12 ni/i in diameter, only slightly 

 larger than the familiar hemoglobin molecule of the chemist and 

 actually considerably smaller than some of the hemocyanin protein 

 molecules. Here, therefore, was a living organism that was smaller 

 than an accepted protein molecule! Was it possible that all the 

 metabolic activities usually associated with living organisms could 

 be packed into a volume no larger than that occupied by a single 

 molecule of the chemist? 



Evidence of a gi'owing unrest and general dissatisfaction with this 

 situation became noticeable in the writings of the time. This is 

 readily understandable since prior to 1935 about all that was really 

 known about viruses was the fact that, in general, they were smaller 

 than bacteria, that they possessed the ability to grow or reproduce 

 within certain kinds of living cells, and that they could mutate or 

 change and adapt themselves to new surroundings. Practically 

 nothing was known about their basic nature. They could not be seen 

 by means of the optical microscope, since, in general, they appeared 

 to be smaller than 300 m/i., which is near the lower limit of tliis instru- 

 ment, and the electron microscope had not yet appeared on the scene. 

 It was not known whether they were still smaller ordinary living 

 organisms or some new type of infectious agent. There was con- 

 siderable discussion regarding their probable nature. Some scientists 



