(I) The simplest and most widely distributed reaction of the host against the parasite is directed by phagocytes, or the white cells of the blood, which are capable of taking up and digesting small parasites. They can also surround a larger parasite and form several layers of cells around its body. Other cells of the tissues often surround the parasites forming thick cysts or capsules, which separate them more or less completely from the rest of the host's body. Inside these capsules or cysts, the parasites often undergo degeneration, and sometimes even calcification (Cysticerci, Trichinella). The cellular reaction may ultimately cause danger ous complications of the disease Bilharziasis. In rats infected with a small round worm, Gongylonema neoplasticum (from cock roaches), the cellular reaction which develops around the worm gives rise to a malignant tumour of the tongue or the stomach.
(2) Humoral reaction (see IMMUNITY) consists in a special alteration in the body of the parasitized host, which becomes in susceptible to infective disease (acquired immunity). It consists in the development of substances (antibodies) in organisms recov ering from infectious diseases, especially if the latter are produced by an infective agent (antigen) such as bacteria and some spiro chaetes. Immunity is less frequent and seldom complete when the disease is caused by animal parasites. It is known, however, in the infections produced by Leishmania (oriental sore), piro plasms, the parasites of malaria and sleeping sickness, and to some extent in cases of infection with various helminths.
The knowledge of the life history of parasites is of paramount importance to man, as it reveals the weak points in their develop ment and adaptations and shows the manner in which infection can be avoided and the parasites attacked and eradicated. The preventive measures directed against parasites fall into three categories: (I) measures against the dissemination of parasites from the infected host ; (2) destruction of parasites inside and outside the host together with destruction of secondary hosts or vectors; and (3) measures to avoid infection.
(I) Dissemination of parasites such as Entamoeba, schisto somes, tapeworms, hookworms and other intestinal forms as well as bacteria causing cholera and typhoid, can be prevented by avoiding the pollution of soil and water by animals and man har bouring the parasites. Dissemination of parasites causing yellow fever, malaria, sleeping sickness and filariasis is prevented by pro tecting the infected hosts from the bites of blood-sucking insects which are the potential carriers of infection.
(2) Destruction of parasites inside the host, which can be at tained by drugs and other treatment, not only cures the host but also diminishes the chances of dissemination of the parasites. The latter can also be attacked outside the body of the host, directly in their free stages, or indirectly by attacking the intermediate hosts such as the fresh-water snail in Bilharziasis. and the blood sucking insects in yellow fever, sleeping sickness and malaria.
(3) Measures to avoid infection naturally vary with the para site involved. Infection with Trichinella, tapeworms and other in testinal parasites, is easily prevented by food inspection and by the thorough cooking of suspected food. Infection with guinea worm is prevented by drinking only filtered water; and boiling all water and vegetables is preventative against such diseases as cholera, typhoid and dysentery. Infection with hookworm and Bilharziasis can be avoided by protecting the skin from contact with contaminated soil or water. Malaria, yellow fever and sleep ing sickness, can be avoided by protection from the bites of the blood-sucking insects which carry these diseases.