The organisms causing the several diseases that I have dealt with in the preceding account fall in three of the four great groups of protozoa, the Infusoria alone being unrepresented. The causes of tropical dysentery of rabies, and possibly of small-pox, are related to the ordinary rhizopod types; the causes of sleeping sickness, trypanosomiasis and syphilis belong to the animal flagellates, and here, possibly, belongs the organism of yellow fever; the causes of malaria, and, probably, of splenomegaly, dum-dum fever and the like belong to the sporozoa.
The effects of these different parasites upon their hosts differ in different cases. Sometimes they poison the host by the liberation of toxins, as in the case of malaria or yellow fever; sometimes by local lesions or general tissue disorganization, as in tropical ulcer, sleeping sickness, syphilis and rabies; or sometimes by the mere mechanical obstruction to normal physiological processes by the accumulation of parasites in capillaries and ducts, while in still other cases the parasites stimulate the latent dividing energy of tissue cells and lead to abnormal tumor-like growths. In many cases knowledge of the organism and of its mode of life has led to preventive measures and to the great saving of human life. In yellow fever, for example, the warfare on mosquitoes will stop entirely its epidemic nature and, thanks to Carroll's brilliant experiments, yellow fever to-day, like malaria or trichinosis, is an advertisement of ignorance or criminal negligence in communities where it exists. Preventive measures hold down the ravages of small-pox, anti-rabic serum lessens the malignancy of hydrophobia, while different specifics are fatal to other kinds of parasitic protozoa, quinine to Plasmodium malariæ, mercury to Treponema pallidum, and recently Koch's specific to Trypanosoma. With the knowledge of these other pathogenic protozoa and of their modes of life investigation will bring out the means of combating them and thus of reducing human suffering, and this prospect if for no other reason, is a full justification of the many commissions that have been appointed, and of the vast sums of money that have been spent, to further protozoan research.
