A disease which is common to man and lower animals is known as a zoo)iosis. Zoonoses were redefined in 1958 by the Joint WHO FAO Expert Committee on Zoonoses as "those diseases and infections which are naturally transmitted between vertebrate animals and man" (World Health Organization, 1959). Less than 20 years ago it was said that there were 50 zoonoses, but in the above report the World Health Organization listed more in connection with vector-borne diseases, altho it need not be restricted to them. epidemiology, taxonomy, evolution, etc.(see ). A dei)ie is a natural population within a species. It lies more or less below the subspecies level, but it is not a formal taxon and is not given a Latin name. There are different types of deme. Nosodemes differ in their clinical manifestations. One example is Leishmania donovani, which has five nosodemes, Indian, Mediterranean, Sudanese, Chinese and South American, which produce different types of disease. Serodemes differ serologically. These are best known among the bacteria and viruses, but also occur among the animal parasites. Tritrichonionas foetus, for example, has several serological types or serodemes. were markedly resistant to infection with Ostertagia circunicincta. while Rambouillets were less so and Southdowns, Shropshires and Hampshires were least resistant.Certain individuals among the more susceptible breeds, however, were just as resistant as the Romneys. Whitlock(1958) has studied genetic resistance to trichostrongylidosis in sheep in some detail.The nutritional status of the host may affect its resistance. Poorly nourished animals are usually more susceptible to infection and suffer more severely from its effects.Protein depletion or protein starvation is particularly important. Lack of specific vitamins and minerals generally decreases resistance, but there are cases in which lack of a certain vitamin which the parasite requires may affect the parasite adversely. Thus, Becker and Smith (1942) found that when calcium pantothenate was added to a ration containing restricted vitamins B,, Bg and pantothenate, the number of oocysts produced hy Einteria nieschulzi infections in the rat was increased.

The subkingdom Protozoa now inclues over 65,000 named species, of which over half are fossil and approximately 10,000 are parasitic. Among living species, this includes approximately 250 parasitic and 11,300 free-living sarcodines (of which approximately 4,600 are foraminiferids); approximately 1,8000 parasitic and 5,100 free-living flagellates; approximately 5,600 parasitic "Sporozoa" (including Apicomplexa, Microspora, Myxospora, and Ascetospora); and approximately 2,5000 parasitic and 4,700 free-living ciliates. There are undoubtedly thousands more still unnamed. Seven phyla of PROTOZOA are accepted in this classification--SARCOMASTIGOPHORA, LABYRINTHOMORPHA, APICOMPLEXA, MICROSPORA, ASCETOSPORA, MYXOSPORA, and CILIOPHORA. Diagnoses are given for these and for all higher taxa through suborders, and reporesentative genera of each are named. The present scheme is a considerable revision of the Society's 1964 classification, which was prepared at a time when perhaps 48,000 species had been named. It has been necessitated by the acquisition of a great deal of nex taxonomic information, much of it through electron microscopy. It is hoped that the present classification incorporatesmost of the major changes that will be made for some time, and that it will be used for many years by both protozoologist and non-protozoologists.

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In 1987, 4516 species and 339 genera of the phylum Apicomplexa had been named. They consisted of the gregarines (subclass Gregarinasida) (1624 named species and 231 named genera), the hemogregarines (family Haemogregarinidae) (399 species and 4 genera), the eimeriorins (order Eimeriorida) (1771 species and 43 genera), the hemospororids (order Haemospororida) (444 species and 9 genera), the piroplasmids (order Piroplasmorida) (173 species and 20 genera), and a few others (105 species and 32 genera). The first apicomplexan protozoon was seen by Antony van Leeuwenhoek; in 1674 he saw oocysts of Eimeria stiedai in the gall bladder of a rabbit. The first member of the phylum to be named (by Dufour in 1828) was Gregarina ovata in earwigs. During the quarter century 1826-1850, 41 species and 6 genera of Apicomplexa were named. These numbers increased progressively. In the quarter century 1951-1975, 1873 new species and 83 new genera were named. Data are given for the numbers of named species and genera of apicomplexan protozoa of each group known in 1850, 1875, 1900, 1925, 1950, 1975, and 1987.