In diseases due to parasites, the parasite produces a change in the tissue in its immediate vicinity often so great as to result in the death of the cells.  The most general direct cause of lesions is toxic or poisonous substances, either introduced from without or formed in the body.  In the case of the parasitic diseases the mere presence of the parasite in the body produces little or no harm, the injury being caused by poisons which it produces, and which act both locally in the vicinity of the parasite and at a distance, being absorbed and entering the blood stream.  How certain of the poisonous substances act is easy to see.  Strong caustics act by coagulating the albumen, or by the withdrawal of water from the cell.  Other poisons act by forming stable chemical compounds with certain of the cell constituents and thereby preventing the usual chemical processes from taking place.  Death from the inhalation of illuminating gas is due to the carbon monoxide contained in this, forming a firm chemical union with the haemoglobin of the red corpuscles so that the function of these as oxygen carriers is stopped.

In order that most poisons may act, it is essential that they enter into the cell, and they cannot do this unless they are able to combine chemically with certain of the cell constituents.  To this is due the selective action of many poisons.  Morphine, for example, acts chiefly on the cells of the brain; strychnine acts on the cells of the spinal cord which excite motion and thus causes the characteristic muscular spasm.  The poisonous substances produced by bacteria, as in the case of diphtheria, act on certain of the organs only.  Different animal species owe their immunity to certain poisons to their cells being so constituted that a poison cannot gain entrance into them; pigeons, for example, cannot be poisoned by morphia.  Individual variations play an important part also; thus, shellfish are poisonous for certain individuals and not so for others.  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.  In the case of poisons the capacity of the cell for adaptation to unusual conditions is of great importance.