Certain animals may lack an appropriate target cell or specific type of cell receptor for the toxin to bind to, and may therefore be nonsusceptible to the activity of the toxin. For example, injection of diphtheria toxin fails to kill the rat. The unchanged toxin is excreted in the urine. There are many reasons why individuals of the same animal species may exhibit greater or lesser susceptibility to the same infective agent. May also be associated with changes in normal flora coincidental to developmental changes in the animal.
Stress is a complex of different factors and apparently has a real influence on health.
Undue exertion, shock, change in environment, climatic change, nervous or muscular fatigue, etc. are factors known to contribute to increases in susceptibility to infection. The best explanation is that in time of stress the output of cortisone from the adrenal cortex is increased. This suppresses the inflammatory processes of the host and the overall effect may be harmful. There are also a number of relationships between stress-related hormones and the functioning of the immune defenses. Also, overfed and obese animals are more susceptible to infection.