There are different kinds of parasites, and the clinical manifestations after invading the human body are different. The main clinical manifestations are: 1. Parasites can seize nutrition, and the nutrients needed for the growth, development and reproduction of parasites in the body mainly come from the organism. The more parasites there are, the more nutrients are taken away, such as roundworms and trichomonads, which take away a large amount of nutrients in the intestine and affect the absorption function of the intestine and cause malnutrition. 2. Parasites can produce damage and compression effects on the parasitized parts and their nearby tissues and organs. Some parasites, especially when they are large and numerous, can cause serious damage, for example, when there are many roundworms, they can twist into clusters and cause intestinal obstruction. 3. The secretions, excretions and decompositions of dead worms have toxic effects on the host. When amoeba in lysed tissue invades intestinal mucosa and liver, it can cause ulceration of intestinal wall and liver abscess.