If the illness does not normally afflict the animal we have to produce it artificially. That is relatively easy in the case of infectious diseases, but it is only apparently easy because there are many pitfalls. Many drugs like digitalis, strophanthin, atropine, are not toxic, on the contrary are beneficial, if they are used in very small doses. On the other hand, any substance, even the most necessary for life, is harmful in excessive doses. Oxygen, for example, when breathed in at a pressure of more than 20 atmospheres kills in a few minutes.
Obesity does, in fact, shorten life.
Apart from dosage, toxicity is linked to time. Thus we distinguish between acute toxicity and chronic toxicity. The main pitfall lies in the fact that each animal species reacts differently to the same infection. Smallpox virus and yellow fever virus do not affect any known animal. But in neither the guinea-pig nor the rabbit has tuberculosis characteristics comparable with tuberculosis in man.