. Biology and human life. Biology. HOW DISEASES ARE CAUSED 303 some cases certain of the substances thus excreted are poisonous to the host. In this way the host may be made seriously ill or even be killed by the poison. Diseases known to be of this kind in man number between thirty and forty, and it is possible that still other ailments may be due to such poisoning. The para- sites causing such disturbances may be plants (mostly bac- teria) or they may be animals (chiefly protozoa). Among the. Fig. 145. The effects of a plant parasite A healthy cotton plant and one attacked by xvilt, a diseas
. Biology and human life. Biology. HOW DISEASES ARE CAUSED 303 some cases certain of the substances thus excreted are poisonous to the host. In this way the host may be made seriously ill or even be killed by the poison. Diseases known to be of this kind in man number between thirty and forty, and it is possible that still other ailments may be due to such poisoning. The para- sites causing such disturbances may be plants (mostly bac- teria) or they may be animals (chiefly protozoa). Among the. Fig. 145. The effects of a plant parasite A healthy cotton plant and one attacked by xvilt, a diseased condition due to the action of a microscopic parasite related to the molds. (From Duggar's "Fungous Diseases of Plants"') diseases caused in this way by bacteria are diphtheria, typhoid fever, and tetanus, or lockjaw. Among the diseases caused in this way by parasitic protozoa are malaria, dysentery, and sleeping sickness. A third effect that parasites may produce upon the host is that of destroying special tissues or organs by using the mate- rial as food (see Fig. 145). The name consumption, which was formerly very common for tuberculosis of the lungs, tells this story strikingly. Parasites that injure chiefly by destroying tis- sues may also give off poisons to some degree, and vice Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Gruenberg, Benjamin C. (Benjamin Charles), 1875-1965. Boston, New York [etc. ] Ginn and Company
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