524 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
and free of all bacteria, had the power of producing the disease when 
rubbed on the leaves of healthy tobacco plants. Despite the evidence 
of this experiment, Ivanovski himself, as well as the scientists of his 
day, believed that the disease was caused by bacteria. 
Seven years later, this phenomenon was rediscovered by Beijerinck, 
who extended these observations and recognized that a new type of 
agent had been discovered. He gave it the appropriate name con- 
tagium vivum fluidum—a living contagious fluid. 
Although the discovery of the existence of viruses is of recent date, 
virus diseases have played a long historic role in the life of man. 
Smallpox existed in China as early as 1700 B.C., and yellow fever was 
recognized as early as the 17th century. In fact, the establishment of 
the Haitian Republic was largely due to yellow fever, since most of 
the French troops sent to invade the island died of the disease there. 
Of course, long before the microbiologic era in medicine, a method 
of preventing one infectious (virus) disease had been devised and its 
use thoroughly established; namely, vaccination against smallpox 
(Jenner). Yet it is curious that, in spite of its ancient lineage, virol- 
ogy has only recently burst forth into full bloom as a vigorous, if not 
explosive, branch of medical science. 
The following pages present a few of the highlights of current ad- 
vances and concepts in virology. 
WHAT IS A VIRUS? 
The submicroscopic particle known as a virus stands in the limbo 
between “living things” and chemical compounds. No longer does 
one ask, “Is a virus a plant or an animal?” Rather, one asks, “Isn’t it 
a complex chemical (a nucleoprotein) with both the actual and po- 
tential properties of life itself?” 
We now know that a virus consists grossly of a protein shell around 
nucleic acid material. It may be, however, that this description is too 
simplified. At any rate, the shell is made up of many small, sym- 
metrically arranged protein molecules. The protein coat may be, like 
most proteins, antigenic. Antibodies produced against this antigen 
can “neutralize” the virus. Thus, antiserum would seem to be useful. 
However, it can act only on virus in serum before it enters a cell; and, 
since this is a transient phase in the life cycle of these tiny particles, 
little clinical use can be made of antiserums (2). 
Inside the protein coat is nucleic acid material which in many 
viruses consists of ribonucleic acid (RNA). Other viruses, includ- 
ing those which attack bacteria (bacteriophage), contain deoxyri- 
bonucleic acid (DNA). In animal tissue, DNA is largely nuclear and 
RNA cytoplasmic in location. 
Nucleic acids are complex chemical molecules. They consist of a 
double spiral of long chains of sugar-phosphate-purine (or pyrimi- 
