In man prolonged fatigue, cold, the use of alcohol to excess and even psychic depression increases susceptibility.

It has been shown that such conditions are accompanied by a diminution in the power of the blood to destroy bacteria.
There is variation in the susceptibility to infection in the different races of man.

If a race be confined to one habitat with close intercourse between the people, such a race may acquire a high degree of immunity to local diseases by a gradual weeding out of the individuals who are most susceptible.

A degree of comparative harmony may be gradually established between host and parasite, as is the case in wild animals.

These have few diseases, the weak die, the resistant breed; they harbor, it is true, large numbers of parasites, but there is mutual adjustment between parasite and host.