Probably the most enduring immunity is in smallpox, although cases are known of two and even three attacks; the immunity is high in scarlet fever, measles, mumps and typhoid fever.

The immunity from diphtheria is short, and in pneumonia, although there must be a temporary immunity, future susceptibility to the disease is probably increased.

In certain cases the immunity is only local; the focus of disease heals because the tissue there has evolved means of protection from the parasite, but if any other part of the body be infected, the disease pursues the usual course.

A boil, for example, is frequently followed by the appearance of similar boils in the vicinity due to the infection of the skin by the micrococci from the first boil, which by dressings, etc., have become spread over the surface.
The natural methods of defence of the host against the parasites have formed the main subject in the study of the infectious diseases for the last twenty years.

Speculation in this territory has been rife and most of it fruitless, but by patient study of disease in man and by animal experimentation there has been gradually evolved a sum of knowledge which has been applied in many cases to the treatment of infectious diseases with immense benefit.