While of course most of these microbes are to be regarded as absolutely harmless and some as very useful, many have the power to do much injury if the proper conditions for their rapid development should at any time exist. While the size of the parasite is always a factor in the damage that it may do to the host the factor of numbers is perhaps of still greater importance because of the power of very rapid multiplication possessed by so many of the smaller forms.
Certain minute parasites in the blood may cause little or no inconvenience, but should they begin to multiply too rapidly some of the capillaries may be filled up and trouble thus result. Or take some of the larger forms. A few intestinal worms may cause no appreciable effect on the host, but as soon as their numbers increase serious conditions may come about simply by the presence of the great masses in the host even if they were not robbing it of its nourishment. Many instances are known where such worms have formed masses that completely clogged up the alimentary canal. Such injuries as these may be regarded as mechanical injuries. Some parasites injure the host only when they are laying their eggs or reproducing the young. These may clog up passages or some of them may be carried to some more sensitive part of the body where the damage is done. The guinea-worm of southwestern Asia and of Africa lives in the body of its host for nearly a year sometimes attaining a great length and migrating through the connective tissue to different parts of the body causing no particular inconvenience until it is ready to lay its eggs when it comes to the surface and then great suffering may result. The African eye-worm is another example of a parasite causing mechanical injury only at certain times. It works in the tissues of the body sometimes for a long while, doing no harm unless it finds its way to the connective tissue of the eyeball.
It is known that many of the germs which cause diseases cannot get into the body unless the protecting membranes have first been injured in some way. Thus the germs that cause plague and lockjaw find their way into the system principally through abrasions of the skin. Many physicians have come to believe that the typhoid fever germ cannot get into the body from the intestine where it is taken with our food or drink unless the walls of the intestine have been injured in some way. It is well known that of the many parasites that inhabit the alimentary canal some rasp the surface and others bore through into the body cavity. This in itself may not be a serious thing, but if the mechanical injury thus caused opens the way for malignant germs, baneful results may follow. Even that popular disease appendicitis is believed to be due sometimes to the injury caused by the work of parasites in the appendix.
Parasites may cause morphological or structural changes in the tissues of their hosts. The stimulation caused by their presence may result in swellings or excresences or other abnormal growths. Interesting examples of this are to be found in the way in which pearls are formed in various mollusks. In the pearl oysters of Ceylon occur some of the best pearls. If these are carefully sectioned there may usually be found at the center the remains of certain cestode larvæ whose presence in the oyster caused it to deposit the nacreous layers that make up the pearl. Other parasites cause similar growths in various shellfish. The great enlargements of the arms or legs or other parts of the body seen in patients affected with elephantiasis is an abnormal growth due to the presence of the parasitic filaræ in some of the lymph-glands where they have come to rest.
Finally, the parasite may exert a direct physiological effect on the host. This is evident when the parasite demands and takes a portion of the nourishment that would otherwise go to the building up of the host. Sometimes this is of little importance, but at other times it may be a matter of life or death to the infected animal. The physiological effect produced may be due to the toxins or poisonous matters that are given off by the parasite while it is living in the host's body. Thus it is believed that the malarial patients usually suffer less from the actual loss of red blood-corpuscles that are destroyed by the parasite than they do from the effects of the poisonous excretions that are poured into the circulation when the thousands of corpuscles break to release the parasites.
One other point in regard to the relation of the parasite to its host and this part of the subject may be dismissed. From our standpoint we look upon the presence of parasites in the body as an abnormal condition. From a biological standpoint their presence there is perfectly normal; it is a necessary part of their life. We think that they have no business there, but from the viewpoint of the parasites their whole business is to be just there. If they are not, they perish. And when we take a dose of quinine or other drug we are killing or driving from their homes millions of these little creatures who have taken up their abode with us for the time being. But they interfere with our health and comfort, so they must go.