The intestine is a rather toxic place for an organism to live, as it is the area where the body tries to break down proteins. Therefore, in order to survive in the gut an organism must have evolved some means to combat the effects of digestive juices. Some simply use a skin like apparatus. Others secrete a protective mucus or toxin.

In addition, everything in the intestine is moving, and in order to remain inside the host an organism must be rather mobile or have evolved a method to anchor itself to the intestinal wall.

Despite these difficulties, the gut is full of nutrients. Both the cells of the intestine and the substances passed through it are commonly full of substances an organism needs. Therefore, the gut is host to a wider variety of parasites (and other organisms) than any other area of the body.