214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
potato yellow dwarf, alfalfa mosaic, curly top of sugar beets, tomato 
spotted wilt, tomato bushy stunt, corn mosaic, cucumber mosaic, and 
sugarcane yellow stripe. Bacteriophages, which are agents capable 
of causing the lysis of bacteria, are now regarded as viruses. 
The viruses have been separated as a special group of infectious, 
disease-producing agents by means of several general properties, no 
one of which is, however, exclusively characteristic of viruses. Never- 
theless, no great amount of difficulty has been encountered in the 
segregation of the virus group. Viruses are characterized by their 
small size, by their ability to reproduce or multiply when within the 
living cells of a given host, by their ability to change or mutate during 
multiplication, and by their inability to reproduce or grow on artificial 
media or in the absence of specific living cells. The sole means of 
recognizing the existence of a virus is provided by the multiplication 
of the virus which is, of course, usually accompanied by manifesta- 
tions of disease. Viruses spread from diseased to normal susceptible 
hosts by different methods. Some are transferred by direct contact, 
as when a diseased leaf is caused to rub against a healthy leaf by a 
gust of wind, or when a normal person or animal comes into direct 
contact with a diseased person or animal. Such viruses can usually 
be spread by indirect contact through the medium of nonspecific 
animate or inanimate objects. Some viruses cannot be transferred 
by direct contact, but require an intermediate host such as a mosquito, 
louse, or leafhopper. Im some cases a highly specific intermediate 
host is necessary, and a more or less definite period of incubation 
within this host may be required before the virus can be transmitted. 
Because properties such as reproduction and mutation have long 
been considered characteristic of living entities, viruses were, for 
many years, regarded as living organisms somewhat smaller than 
ordinary bacteria. However, the isolation in 1935 of tobacco mosaic 
virus in the form of a crystalline nucleoprotein of unusually high 
molecular weight and the subsequent isolation of still other viruses 
in the form of high molecular weight nucleoproteins, some of which 
were also crystallizable, cast doubt upon the validity of classifying all 
viruses as organisms. With the exception of virus activity, the 
properties of some of the smaller viruses are quite similar to the 
properties of ordinary protein molecules, whereas at the other extreme 
with respect to size, the properties of the viruses are more nearly like 
those of accepted living organisms. The viruses, therefore, serve as a 
bridge between the molecules of the chemist and the organisms of 
the bacteriologist, and provide us with new reasons for considering 
that life, as we know it, owes its existence to structure, to a specific 
state of matter, and that the vital phenomenon does not occur spon- 
taneously, but is possessed in varying degrees by all matter. It is 
