Owing to the variability of living structures a substance may be poisonous at one time and not at another, as the following example shows.

A man, very fond of crab meat, was once violently poisoned after eating crabs, being at that time seemingly in his usual state of health, and no illness resulted in others who had partaken of the same crabs.

Two months later a hearty meal of crabs produced no ill result.

There are also individuals so constituted that so simple a food as the egg is for them an active poison.
The lesions produced by the action of injurious conditions are usually so distinctive in situation and character that by the examination of the body after death the cause of death can be ascertained.

The lesions of diseases may be very obvious to the naked eye, or in other cases only the most careful microscopic examination can detect even the presence of alterations.