VIROLOGY—LILLY RESEARCH LABORATORIES 525 
dine) groups; one such unit is called a nucleotide. RNA differs from 
DNA in the sugar moiety. The complexity of nucleic acids can be 
judged from the fact that thymus DNA has a molecular weight on 
the order of 4,000,000 and contains about 10,000 nucleotides. 
Infectivity and the capacity to damage or destroy cells are said to 
be functions of the nucleic-acid content. 
Viruses vary considerably in size and, to some extent, in shape. The 
largest measure about 300 millimicrons (mp) in diameter, the smaller 
about one-tenth of this or less. (In comparison, a staphylococcus has 
a diameter of about 1,000 my.) Their configuration, as determined 
from advanced technics, may be classified roughly into spheres, rods, 
and ellipsoids (pl. 1). 
CLASSIFICATION 
No satisfactory classification comparable to bacterial groupings has 
been devised for viruses. These individualistic creatures do not seem 
to fall into a logical pattern. They may be grouped according to their 
primary site of action, antigenic relationship, mode of transmission, 
pathologic tissue reactions, immunologic properties, etc. Because we 
know a lot more about the diseases than about the organisms, the simple 
“clinical” classification provided in table 1 may be useful. 
TABLB 1.—Clinical grouping of selected virus diseases 
Dermatotropic: Arthropod-borne: 
Smallpox Yellow fever 
Chickenpox Dengue 
Herpes zoster Encephalitides 
Measles 
Psittacosis—Lymphogranuloma Group: 
Herpes simplex Ue Se y 
Psittacosis 
Exanthem subitum 
i zs Lymphogranuloma venereum 
Respiratory—Parotid: 
Trachoma 
Influenza : : ratty: 
Inclusion conjunctivitis 
Mumps 
Adenovirus infection Miscellaneous: 
“Common cold” Coxsackie virus infection 
Neurotropic : ECHO virus infection 
Poliomyelitis Verrucae 
Rabies Epidemic keratoconjunctivitis 
Lymphocytic choriomeningitis Foot-and-mouth disease 
Hepatic: Salivary gland virus infection 
Infectious hepatitis 
Serum hepatitis 
MULTIPLICATION OF VIRUSES 
It may come as a surprise to know that invasion by and multiplica- 
tion of viruses are known in considerable detail. Some description of 
these processes is essential for a fundamental appreciation of the 
problems at the bedside. 
Bacterial viruses.—Bacteriophage (phage, for short) has a spe- 
cialized head and tail structure. Phage attaches itself to the surface 
