Parasites parasitize in the cells, tissues or cavities of the host, causing a series of injuries, which are not only seen in protozoa, adult worms, but also in migrating larvae. Their effects on the host are various. (1) The substances needed for the growth, development and reproduction of parasitic worms in the host mainly come from the host, and the more parasitic worms there are, the more nutrients will be taken. For example, ascaris lumbricoides and tapeworms parasitize in the intestine, seize a lot of nutrients, and affect the intestinal absorption function, causing malnutrition in the host; Another example is hookworm attached to the intestinal wall to absorb a lot of blood, which can cause anemia in the host. (2) Mechanical damage parasites can damage or oppress the parasitic parts and their nearby tissues and organs. Some parasites, especially when the individual is large and the number is large, this harm is quite serious. For example, ascaris lumbricoides can twist into a ball and cause intestinal obstruction. Echinococcus is parasitic in the liver. At first, it has no obvious symptoms. Later, it grows up and oppresses the liver tissue and other organs in the abdominal cavity, resulting in obvious symptoms of oppression. In addition, the migration of larvae in the host body can cause serious damage. For example, ascaris lumbricoides larvae pierce the capillaries in the alveolar wall when they migrate in the lung, which can cause bleeding. (3) Toxicity and the role of antigen substances. The secretions, excreta and decomposition products of dead worms of parasites have toxic effects on the host, which is the most important type of parasites endangering the host. For example, when Entamoeba histolytica invades intestinal mucosa and liver, it secretes histolytic enzyme, which dissolves tissues and cells, causing intestinal wall ulcer and liver abscess of the host; The excreta secreted by Schizocephala platyphylla may affect the hematopoietic function of the host and cause anemia. In addition, the metabolites of parasites and the decomposition products of dead worms are antigenic, which can sensitize the host and cause local or systemic allergic reactions. For example, the secretion of miracidium in Schistosoma eggs causes immunopathological changes in the surrounding tissues-egg granuloma, which is the most basic pathological change and the main pathogenic factor of schistosomiasis. Another example is that the antigen of plasmodium forms an immune complex with the corresponding antibody, which is deposited on the basement membrane of glomerular capillary, and with the participation of complement, it causes glomerulonephritis. As well as the rupture of Echinococcus cyst wall and the entry of cyst fluid into abdominal cavity, it can cause allergic shock and even death of the host. (4) Hypersensitivity parasites often induce hypersensitivity in the host and cause tissue damage. This is one of the pathogenic effects of parasites. Hypersensitivity is generally divided into four types, that is, I, II, III and IV. The first three types are antibody-mediated hypersensitivity, and the fourth type is mainly T-cell and macrophage-mediated hypersensitivity. These four kinds of hypersensitivity reactions are also called immediate hypersensitivity, cytotoxic hypersensitivity, immune complex hypersensitivity and delayed hypersensitivity respectively.