C1. _Schizonts._ 1. Normal red cell. 2, 3, 4, 5, 6. Young ring forms. These are hair-like rings and are the only forms besides crescents to be found in the peripheral blood. _In very heavy infections_ or in smears from spleen the following forms are found. 7. Beginning division of chromatin. 8 and 9. Further division. 10. Merocyte. C2. _Female gametes._ 1 and 2. Young macrogametes. 3. Older stage. 4. Development in red cell. 5 and 6. Fully developed female crescents showing clumping of pigment and rich blue colour. _Male gametes._ 1 and 2. Developing forms. 3 and 4. Fully developed microgametocytes. 5. Flagellated body developed in wet preparation. [Illustration: PLATE OF MALARIAL PARASITES] SECTION I DISEASES DUE TO PROTOZOA CHAPTER I MALARIA DEFINITION AND SYNONYMS =Definition.=—Malaria is a protozoal disease caused by three species of _Plasmodium_. In the clinically benign types of malaria we have that of benign tertian, due to _P. vivax_, with a tertian periodicity and that of quartan, due to _P. malariae_ and showing a quartan or seventy-two hour periodicity. The clinically malignant type of malaria is due to _P. falciparum_, the parasite of malignant tertian or aestivo-autumnal malaria. The benign malarial fevers are characterized by a frank chill and well marked distinctions of cold, hot and sweating stages. In malignant tertian there is an indefinite or dumb chill with prolonged hot stage. Diagnostic of malaria are periodicity, parasites and splenic enlargement. The malignant tertian parasite is the one responsible for the so-called cerebral and algid manifestations of perniciousness. Man is the intermediate host of the parasite while the sexual cycle or sporogony goes on in some species of mosquito of the anopheline subfamily, the definitive host. =Synonyms.=—Remittent Fever, Intermittent Fever, Ague, Marsh Fever, Paludism, Jungle Fever. French: Paludisme. German: Wechselfieber. HISTORY AND GEOGRAPHICAL DISTRIBUTION =History.=—Hippocrates, who considered malaria as intimately connected with bile, divided the disease into quotidian, tertian and quartan, differentiating such types of fever from continuous fevers. It is interesting to note that Celsus recognized two types of tertian fever, the one benign and similar to quartan fever, the other far more dangerous, with a fever occupying thirty-six of the forty-eight hours, not entirely subsiding in the remission, but being only mitigated. In the time of Caesar views were expressed by Varro that swamp air might be the cause of malaria and furthermore that animals, so small that the eye could not follow them, might transmit diseases by way of the mouth or nose.