Commensal means a microorganism that normally lives in close contact with a human or animal without causing any harm. This would include many of the bacteria normally present on a person's or animal's skin or intestine.
Commensal is applied to pests that eat the same food as another. The term is often applied specifically to rodent pests of the landscape, that eat human food. House mice and rats are the chief commensal rodents. In an integrated pest management system (IPM), commensal pests will be discouraged from arriving by making sure food is kept in well-sealed containers. One of the principles of IPM is that it is preferable to take preventive measures against the arrival of commensal rodents and other pests, rather than having to fumigate commensal pests or otherwise exterminate them later