Coming now to the minute, microscopic, one-celled animals, the Protozoa, we find entire groups of them that are living parasitic lives, depending wholly on one or more hosts for their existence. Many of these have a very remarkable life-history, living part of the time in one host, part in another. The malarial parasite and others that cause some of the diseases of man and domestic animals are among the most important of these.
PARASITISM
Among all these parasites, from the highest to the lowest the process that has fitted them for a parasitic life has been one of degeneration. While they may be specialized to an extreme degree in one direction they are usually found to have some of the parts or organs, which in closely related forms are well developed, atrophied or entirely wanting. As a rule this is a distinct advantage rather than a disadvantage to the parasite, for those parts or organs that are lost would be useless or even in the way in its special mode of life.
Then, too, the parasite often gives up all its independence and becomes wholly dependent on its host or hosts not only for its food but for its dissemination from one animal to another, in order that the species may not perish with the host. But in return for all this it has gained a life of ease, free from most of the dangers that beset the more independent animals, and is thus able to devote its whole time and energy to development and the propagation of the species.
We are accustomed to group the parasites that we know into two classes, as harmful or injurious and as harmless, the latter including all those kinds that do not ordinarily affect our well-being in any way. But such a classification is not always satisfactory or safe, for certain organisms that to-day or under present conditions are not harmful may, on account of a great increase in numbers or change of conditions, become of prime importance to-morrow. An animal that is well and strong may harbor large numbers of parasites which are living at the expense of some of the host's food or energy or comfort, yet the loss is so small that it is not noticed and the intruders, if they are thought of at all, are classed as harmless. Or we may at times even look upon them as beneficial in one way or another. "A reasonable amount of fleas is good for a dog. They keep him from brooding on being a dog."
But should these parasites for some reason or other increase rapidly they might work great harm to the host. Even David Harum would limit the number of fleas on a dog. Or the animal might become weakened from some cause so that the drain on its resources made by the parasites, even though they did not increase in numbers, would materially affect it.
Perhaps the most serious way in which parasites that are usually harmless may become of great importance is illustrated by their introduction into new regions or, as is more often the case, by the introduction of new hosts into the region where the parasites are found. Under normal conditions the animals of a given region are usually immune to the parasites of the same region. That is, they actually repel them and do not suffer them to exist in or on their bodies, or they are tolerant toward them. In the latter case the parasites live at the expense of the host, but the host has become used to their being there, adapted to them, and the injury that they do, if any, is negligible.
But when a new animal comes into the region from some other locality the parasites may be extremely dangerous to it. There are many striking examples of this. Most of the people living in what is known as the yellow fever belt are immune to the fever. They will not develop it even under conditions that would surely mean infection for a person from outside this zone. Certain of our common diseases which we regard as of little consequence become very serious matters when introduced among a people that has never known them before. The cattle of the southern states are immune to the Texas fever, but let northern cattle be sent south or let the ticks which transmit the disease be taken north where they can get on cattle there, and the results are disastrous.
Another striking example and one that is attracting world-wide attention just now is the trypanosome that is causing such devastation among the inhabitants of central Africa. With the advent of white men into this region and the consequent migration of the natives along the trade routes this parasite, which is the cause of sleeping sickness, is being introduced into new regions and thousands upon thousands of people are dying as a result of its ravages.
DISEASES CAUSED BY PARASITES