TRANSMISSION AND SPREAD OF VIRUSES 9 
crops like potatoes, strawberries, raspberries, and hops are of such 
importance. 
Air-borne Transmission, including Droplet Infection 
It seems likely that air-borne infection does not play a major part 
in the dissemination of viruses, but this method of transmission may 
be important in certain cases. 
Air-borne infection may be conveyed in large droplet projectiles 
sprayed short distances from the mouth or nose. Furthermore, 
similarly expelled droplets o-1 mm in diameter or less will have 
become so small through evaporation before they reach the floor that 
as “droplet nuclei” they can float in the air for many hours or even 
days. These droplet nuclei may play an important part in the trans- 
mission of some viruses. Influenza virus, for example, on reaching 
the floor, bedclothes, and other objects, may survive on dust particles 
and these may be subsequently re-suspended in the air. It has been 
demonstrated that influenza virus of the PR 8 strain could be re- 
covered from dry dust exposed near a ferret infected with influenza. 
Between 1 and 10 per cent of virus would withstand drying in house- 
hold dust and the time the virus will remain viable under these con- 
ditions has also been demonstrated. Thus there seems to be little 
depreciation after three days, whilst 10 per cent may persist for a 
week, and 1 per cent for a fortnight. 
‘Dried epidermal scales from cases of smallpox are regarded as 
infective and their aerial transfer may carry the infection to a distance. 
There is also the possibility whether the virus of the common cold 
might not be shaken out of handkerchiefs and so transmit the infection 
through the air for short distances. This is a question of considerable 
importance, and work now in progress suggests that the impregnation 
of handkerchiefs with a disinfectant might make them less dangerous. 
Recent experiments have shown that the viruses of poliomyelitis, 
influenza, and laryngotracheitis can all be transmitted by the exposure 
of susceptible animals to artificially contaminated air. It has now been 
shown that the pneumoencephalitis virus of fowls (Newcastle disease 
or fowl-pest) can be recovered from the air of poultry houses con- 
taining infected birds: air from an infected house sampled in 540- or 
1080-litre quantities contained virus in sufficient concentration to 
infect chick embryos. In an attempt to test the infectivity of such air 
for chickens, four normal chicks were exposed to the aerial environ- 
ment of a house containing birds affected with pneumoencephalitis. 
