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.