The protozoa are unicellular (single celled) animals, which can complete various physiological functions one their own. All those parasitizing human body are microscopic in size, from 2~200 μm. About 40 species are relative to human diseases, which are called medical protozoa.

  I. Basic structure
  1. The cell membrane is the bio-membrane described by the fluid mosaic model（液态镶嵌模型）. It has the functions of sensation, recognition, taking food, material exchange, locomotion and pathogenisis. There are many accessory structures on it, such as ligand, receptor, carrier, enzyme, antigen and toxin on it, some of which are the material base leading to disease.
  2. The cytoplasm is differentiated into ectoplasm and endoplasm. The ectoplasm is a hyaline outer layer that is protective in function and also gives rise to the locomotive organ, such as pseudopodium, flagella and cilia. The endoplasm is the inner position where there are various organelles and food vacuoles. It manages the metabolism.
  3. The nucleus is the most important organelle, which controls the metabolism, heredity and reproduction. If nucleus is injured the cell will soon die. There are two types of nuclei, vesicular and compact. The morphological feature of the nucleus is used to identify the kind of protozoa.

  II. Life cycle
  1. Trophozoite is a living stage of protozoa when they can move, take food and reproduce. (It is usually the pathogenic stage.)
  2. Cyst is the resting stage of a protozoa with a protective wall. It is usually the infective stage. Its functions are protection, transmission and multiplication.
  3. Site of inhabitation: digestive tract; urogenital canals; blood and tissues
  4. Infective route: mouth; direct or indirect contact; sexual transmission; placenta; insect sucking blood, blood transfusion and breath.

  III. Reproduction
  1. Asexual reproduction
  (1) Binary fission is the simplest form of division. The organism is transversely or longitudinally divided into two daughter parasites.
  (2) Multiple fission (schizogony): First, multiple division of the nucleus takes place and then each nucleus is surrounded by a portion of cytoplasm. Finally, many daughter cells will be formed
  (3) Endodyogeny: a cell undergoes a single internal budding and then two daughter cells are produced, such as Toxoplasma gondii.
  2. Sexual reproduction
  (1) Conjugation: two cells temporarily attach to each other, exchange their nuclear material and then separate, such as Balantidium coli.
  (2) Gametogony (syngamy): two sexually differentiated cells unite to form the zygote and then produce many daughter cells, such as the gametogony of Plasmodium vivax.
  3. Alternation of generation: In life cycles of some protozoa, there is the regular alternation of sexual and asexual reproductions , this phenomenon is called alternation of generation, such as it in the life cycle of Plasmodium vivax.

  IV. Pathogenic mechanism
  1. Parasites massively multiply and cooperate with bacteria in pathogenesis, such as Trichomonas vaginalis.
  2. Parasites massively multiply and destroy the cells and tissues of the host, such as Plasmidium vivax.
  3. Parasites massively multiply and invade the adjacent tissues, such as Entamoeba histolytica.
  4. Intracellular parasites are carried to all parts of the body by blood stream.
  5.Opertunistic protozoa: the protozoan living in the human body in commensalisms make the host attack when his immunity is lower or restrained,such as Pneumocystis carinii and Toxoplasma gondii.

   V. Classification:
  The classification of protozoa mainly depends on their locomotive mode.
  1.Class Zoomastigophora: Leishmania Donovani moves by flagellum.
  2.Class Lobosea: Entamoeba histolytica moves by pseudopodium.
  3.Class Sporozoa: Plasmodium vivax
  4.Class Kinetofragminophorea: Balantidium coli move by cilia