The first person to see protozoa was the Dutch microscopist, Antony van Leeuwenhoek (1632-1723). He used simple lenses which he ground himself and which gave magnifications as high as 270 times. His letters to the Royal Society are a classic of biology. Between 1674 and 1716, Leeuwenhoek described many free-living protozoa, among them, according to Dobell (1932), being Euglena, Volvox and Vorticella. Huygens in 1678 was the first to describe Paramecium. Classic work on free-living protozoa was done by O. F. Muller (1786), Ehrenberg (1830, 1838) and Dujardin (1841).

The first parasitic protozoon to be discovered was Eimeria stiedae; Leeuwenhoek found its oocysts in the gall bladder of an old rabbit in 1674. Later, in 1681, Leeuwenhoek found Giardia lamblia in his own diarrheic stools, and in 1683 he found Opalina and Nyctotherus in the intestine of the frog.

The first species of Trichomonas, T. tenax, was found by O. F. Muller in 1773 in the human mouth; he named it Cercaria tenax. Donne found T. vaginalis in the human vagina in 1837, and Davaine found Trichomonas and Chilomastix in the stools of human cholera patients in 1854.

The first trypanosome was discovered in the blood of the salmon by Valentin in 1841, and the frog trypanosome by Gluge and Gruby in 1842. Lewis found the first mammalian trypanosome, T. lewisi, in the rat in 1878. Evans discovered the first pathogenic one, T. evansi, in 1881 in India, where it was causing the disease known as surra in elephants. Bruce discovered T. brucei in Africa in 1895 and described its life cycle and transmission by the tsetse fly in 1897. In 1902, Dutton discovered that African sleeping sickness of man was caused by T. gambiense. Leishmania tropica was first seen by Cunningham in India in 1885 and was first described and identified as a protozoon by Borovsky in Russia in 1898. Leishman and Donovan independently discovered Leishmania donovani in India in 1903.

Histomonas meleagridis, the cause of blackhead of turkeys, was discovered by Theobald Smith in 1895. Its transmission in the eggs of the cecal worm was discovered by Tyzzer and Fabyan in 1922 and described in detail by Tyzzer in 1934.

The first parasitic amoeba, Entamoeba gingivalis, was found in the human mouth by Gros in 1849. Lewis found E. coli in India in 1870, and Losch found E. histolytica in Russia in 1875.

Balantidium coli was discovered by Malmsten in 1857.

It was not until 154 years after Leeuwenhoek saw Eimeria stiedae that any other telosporasids were found. Then, in 1828, Dufour described gregarines in the intestines of beetles, and in 1838 Hake rediscovered the oocysts of E. stiedae. The most extensive early study of the coccidia was that of Eimer (1870), who described a number of species in various animals. Schaudinn and Siedlecki (1897) described the gametocytes and gametes of coccidia and showed that they formed zygotes. Further studies on the life cycle of coccidia were published by Schaudinn in 1898 and 1899. Classic work on the coccidia of gallinaceous birds was done by Tyzzer (1929) and Tyzzer, Theiler and Jones (1932).

The human malaria parasite was discovered in 1880 by the French army doctor, Alphonse Laveran. Golgi (1886, 1889) reported on its schizogony and distinguished the types of fever caused by the different species. MacCallum (1897), working with the closely related Haemoproteus of birds, recognized that the exflagellation which had been seen by Laveran was microgamete formation, and later observed fertilization and zygote formation in Plasmodium falciparum.

Ross worked out the life cycle of the bird malaria parasite, Plasmodium relictum (P. praecox), in India in 1898, showing that it was transmitted by the mosquito, Culex fatigans. Working independently in Italy, Grassi and his collaborators (1898) almost immediately afterward found that human malaria is transmitted by Anopheles mosquitoes.

Babesia bovis was discovered by Babes in 1888. Theobald Smith and Kilborne described the cause of Texas fever of cattle, B. bigemina in 1893; they showed that it was transmitted by the tick, Boophilus annulatus, being passed thru its eggs to the next generation of ticks which then infected new cattle. This was the first demonstration of arthropod transmission of a protozoon.

The present century has seen many advances in protozoology, but there are many more ahead. Several times more species of parasitic protozoa have been described since 1900 than were known before, but these are only a fraction of the total number. Exciting new discoveries are being made every year on the physiology and nutritional requirements of protozoa (Lwoff, 1951; Hutner and Lwoff, 1955), and the life cycles, host-parasite relations, and pathogenesis of many species are only now being worked out. The electron microscope and the phase microscope have opened up a whole new field for morphologic study, chemotherapy is progressing rapidly, and new discoveries are being made even in taxonomy, which most people used to consider a dead field.