This is the case in typhoid fever, malaria and yellow fever.

The comparative immunity of the natives to yellow fever in regions where this prevails seems to be due to their having acquired the disease in infancy in so mild a form that it was not recognized as such.
The infectious diseases are preeminently the diseases of the first third of life.

After the age of forty man represents a select material.

He has acquired immunity to many infections by having experienced them.

Habits of life have become fixed and there is a general adjustment to environment.