G . H. F. Nutt all 
75 
of P. humanus, the disease being confined essentially to cool and tem¬ 
perate climates 1 . 
The virus of typhus occurs in the blood of affected individuals. 
Blood collected on the 3-10tb day of the attack has been found to be 
virulent. The evidence as to the nature of the virus is still contra¬ 
dictory. The virus can be maintained in undiminished virulence by 
passage through animals (chimpanzee, monkey, guinea-pig). 
Infection occurs through the bite of infective lice or through such 
lice being crushed upon the skin when the latter is scratched. Lice 
which have sucked blood containing the virus are capable of producing 
infection during 7-11 days after they have infected themselves. If such 
lice are crushed 9-10 days after an infective feed, or if their faeces are 
collected 3-6 days after they have fed on infective blood, their contents 
and faeces respectively are capable of producing infection if placed upon 
the excoriated skin. It has not been determined how long lice remain 
infective when once contaminated. The evidence regarding the here¬ 
ditary transmission of the virus in the louse is contradictory. 
Lice are invariably present in connection with typhus outbreaks. 
The destruction of lice upon a typhus patient renders him innocuous. 
The prophylaxis of typhus consists in louse destruction. Bedbugs 
and fleas do not convey the disease. 
Recent experience is in agreement with observations, dating from 
1739, in respect to the occurrence of typhus and relapsing fever side by 
side in epidemic form. 
Relapsing fever. 
Epidemiological evidence, as in the case of typhus, points to relapsing 
fever being louse-transmitted. The two diseases occur under like con¬ 
ditions : personal filth and squalor, the close contact of persons through 
overcrowding, facilities being offered for the propagation of lice by the 
continuous wearing of clothing day and night for weeks or months 
on end. All recently collected evidence (from India, North Africa, 
1 This appears to me to be due to the relatively restricted number of lice present on 
the populations of warm countries owing to the insects being largely confined to the head, 
the light garments or slight clothing worn affording unsuitable conditions for a con¬ 
siderable propagation of lice upon the body, and the high temperature to which they are 
exposed in proximity to the body in warm countries affecting the body-lice adversely. 
The louse population per man is usually much greater when the body is infested than 
when the head is affected, and the chances of lice finding a fresh host are greater among 
heavily clothed populations. The factor of temperature and its possible influence on the 
virus in the louse must likewise be taken into account. (See under Biology of Pediculus 
humanus pp. 80 et seq.) 
