Distribution of Tropical Diseases in Africa. 477 
of birth. Although no absolute immunity is acquired by 
acclimatisation, yet it is true that a certain amount of 
immunity is possessed by those who have lived for a con- 
siderable period in any locality constantly or frequently 
visited by the disease. The chances of immunity appear to 
be always in direct proportion to the lengthened residence 
at the headquarters of the disease, but no protection is 
acquired except by those who have passed through a 
previous epidemic period without quitting the country. 
Any benefit, however, gained by acclimatisation is imme- 
diately lost on change of residence, even though that change 
be to a healthier locality. This remark applies equally to 
negroes and white men, but negroes suffer far less from 
yellow fever than do the whites. 
The nature of the yellow fever poison has been found by 
Dr Domingos Freire to be a specific cryptococcus. He has 
also found out that this micro-organism secretes an alkaloid 
resembling a ptomaine, which acts as a violent poison. With 
regard to contagion, there is no doubt that it may be con- 
veyed by fomites or merchandise, as also by ships, and it 
can be transported farther by sea than on land. Hence the 
necessity during an epidemic of completely isolating vessels 
ina harbour. It does not appear that contact with the sick 
has power to spread the disease. Electricity appears to 
have a singular and baneful influence upon persons suffering 
from yellow fever. 
It is impossible in the space at my disposal to detail the 
symptoms of yellow fever, but there are a few well-marked 
symptoms which deserve notice. First, the attack is sudden, 
there is a want of correlation between the pulse and tem- 
perature, albuminuria is invariably present, the patient 
suffers from the black vomit, from a general hemorrhagic 
tendency, and from a yellow discoloration of the skin, often, 
too, from suppression of urine. 
Prophylactic Treatment—Avoid the yellow-fever season if 
possible, namely, the hot and rainy season; avoid chills by 
wearing proper clothing; avoid exposure to the sun as much 
as possible. Individuals attacked by the disease should be 
isolated. Residences should be chosen at the highest 
