Even in the most severe epidemics all are not equally affected, some escape the infection, others have the disease lightly, others severely, some die.
Chance enters into this, but plays a small part, for the same varying individual susceptibility is shown experimentally.

If a given number of animals of the same species, age and weight, even those from the same litter, be inoculated with a given number of bacteria shown to be pathogenic for that species, the results differ.

If the dose be necessarily fatal, death will take place at intervals; if a dose smaller than the fatal be used, some animals will die, others will recover.

The defences of the organism being centred in the activity of the living tissue, any condition which depresses cell activity may have an effect in increasing susceptibility to infection.

Animals which ordinarily are not susceptible to infection with a certain organism may be made so by prolonged hunger, or fatigue, by the influence of narcotics, by reduction of the body temperature, by loss of blood.