Malaria differs from most germ diseases in the fact that the organisms which produce it are not eliminated from the body in any way.  In most germ diseases the germs are discharged from the patient by secretions or excretions of some kind, and from these excretions may readily find their way into other individuals.  The malarial organism is not discharged from the body in any way, and hence is not contagious.  If the parasite does pass part of its history in some other animal than man, there must be some means by which it passes from man to its other host.  It has been suggested that some of the insects which feed upon human blood may serve as the second host and become inoculated when feeding upon such blood.  This has been demonstrated with startling success in regard to the mosquito (Anopheles), some investigators going so far as to say that this is the only way in which the disease can be communicated.

Several other microscopic animals occur as parasites upon man, and some of them are so definitely associated with certain diseases as to lead to the belief that they are the cause of these diseases.  The only one of very common occurrence is a species known as Amaeba coli, which is found in cases of dysentery.  In a certain type of dysentery this organism is so universally found that there is little doubt that it is in some very intimate way associated with the cause of the disease.  Definite proof of the matter is, however, as yet wanting.

On the side of plants, we find that several plants of a higher organization than bacteria may become parasitic upon the body of man and produce various types of disease.  These plants belong mostly to the same group as the moulds, and they are especially apt to attack the skin.  They grow in the skin, particularly under the hair, and may send their threadlike branches into some of the subdermal tissues.  This produces irritation and inflammation of the skin, resulting in trouble, and making sores difficult to heal.  So long as the plant continues to grow, the sores, of course, can not be healed, and when the organisms get into the skin under the hair it is frequently difficult to destroy them.  Among the diseases thus caused are ringworm, thrush, alopecia, etc.

CHAPTER VI.

Methods of combating parasitic bacteria.

The chief advantage of knowing the cause of disease is that it gives us a vantage ground from which we may hope to find means of avoiding its evils.  The study of medicine in the past history of the world has been almost purely empirical, with a very little of scientific basis.  Great hopes are now entertained that these new facts will place this matter upon a more strictly scientific foundation.  Certainly in the past twenty-five years, since bacteriology has been studied, more has been done to solve problems connected with disease than ever before.  This new knowledge has been particularly directed toward means of avoiding disease.  Bacteriology has thus far borne fruit largely in the line of preventive medicine, although to a certain extent also along the line of curative medicine.  This chapter will be devoted to considering how the study of bacteriology has contributed directly and indirectly to our power of combating disease.