The action of heat upon the life of protozoan and metazoan parasites and all forms of life in general is such that slight variations in temperature suffice to produce profound changes in their life history.
Thus, the human or bovine type of tubercle bacillus requires a constant temperature between 38° and 39°C. for its development in artificial culture; and a slight variation above or below this point arrests its growth. This probably explains why this type of tubercle bacilli are non-pathogenic and apparently harmless when inoculated into birds which have a temperature above 40°C. or when inoculated into cold-blooded animals that have a lower temperature than man and most mammals. The same is true of most pathogenic bacteria. The very interesting experiments carried on by Pasteur with anthrax bacillus clearly elucidates the importance of temperature in infection. Thus anthrax bacillus which is commonly pathogenic for man and most mammals, and non-pathogenic for birds can be successfully inoculated into hens by lowering their body temperature.