The fimbriae of bacteria allow the bacterium to atach to host cells, such as in the intestinal epithelium (the cells lining the inside of the intestive of the gut). They usually attach to carbohydrate parts of glycoprotiens/glycolipids in the membrane of the host's cells. Other bacteria have proteins on their own membrane that attach to host molecules called cell adhesion molecules (CAMs) that normally mediate cell-cell interactions and extracellular matrix actions. The bacterial proteins mimic these host proteins and allow them to attach via these. Most CAMs are integrins, cadherins, IgCAMs or selectins, each being favoured by different species. For example, Yersinia bacteria often use integrins (yersinia is assumed to have caused the plague by most). Helicobater pylori targets selectins, and are responsible for a large proportion of gastric ulcers and cancers. The glycocalyx is a netowrk of polysaccharides that performs the same job, allowing it to attach to host cells. This process of adhesion to host cells allows the bacteria to overcome mechanical removal. For example, due to muscular activity, the contents of the gut is always moving (albeit very slowly sometimes) and so the bacteria need to have a mechanism of attaching to a host cell to allow them to infect the host, otherwise they would simply be excreted out. A similar need is in the lungs, since the constant motion of air would just blow the bacteria away if it didn't attach to host cells.