CHAPTER II 
TRANSMISSION AND SPREAD OF VIRUSES 
THE essence of a virus disease is its infectious nature, so that all known 
viruses are transmissible in one way or another, but the degree of 
infectiousness differs in the different viruses. It follows, therefore, 
that the natural mode of spread also differs. In this chapter both 
natural and artificial means of transmission are considered, but the 
relationship between viruses and their arthropod vectors is too 
important to be included here except briefly, and it is dealt with more 
fully in a subsequent chapter. The various methods of spread, so far 
as they are known, are dealt with in turn. 
Mechanical Contamination, Direct and Indirect 
By direct contamination is meant the actual contact between the 
infected and the healthy organism while indirect contamination 
involves the spread of the virus by means of infected material, such 
as saliva, feathers, fomites, etc. 
One of the most infectious viruses is that of foot-and-mouth 
disease, which is spread by direct and indirect contact. The saliva, 
urine, and milk of infected cattle are all infectious before the appearance 
of any symptoms, and it is probable that much spread of the disease 
takes place by indirect contact. When dried on certain materials the 
virus can remain infective for several weeks; for example, hay 
treated with infective saliva was capable, a month later, of transmitting 
the disease to a calf which ate it. 
The following incident, recorded by Muessemeier, illustrates the 
ease of spread of this virus by indirect contact. An outbreak of 
“bovine pest” in Silesia was isolated by a double ring of military 
posts which stopped all traffic, human and animal. However, twenty- 
three days after the establishment of the isolation an outbreak of 
foot-and-mouth disease developed in the isolated village. Investi- 
gation showed that a carter, coming from a village twelve miles away 
where there was a case of foot-and-mouth disease, had slipped through 
the military cordon during the night by creeping along a dry ditch, 
in order to pay a clandestine visit to a girl. He remained at the house 
a few hours, returning by the same route. The next morning the 
5 
