The relationship between a flea and a mouse is classified as parasitism. Of all mammals, rodents are the most heavily parasitized by fleas, according to Encyclopedia Britannica. Flea larvae develop easily in the nests that rodents, such as mice, build inside holes.
A parasitic relationship is defined by an organism acting as a parasite by living off a host organism, bringing it harm and potentially leading to the host's death. Fleas feed on the blood of their hosts, leading to itchy sores, and live in their hosts' fur. Fleas provide no benefit whatsoever to their hosts.
Fleas even transmit a variety of other parasites and diseases, such as tapeworms, bubonic plague, endemic typhus and myxomatosis. In extreme cases, the flea bites themselves, which can cause anemia. This is especially true in small animals, such as mice. Fleas may also lead to loss of fur as a result of the animal biting and scratching itself to soothe itches.
Fleas also use their hosts to move from one place to another. According to Ransford Environmental Solutions, flea-infested mice occasionally enter human homes. This can quickly lead to an out-of-control infestation of both mice and fleas. A warm and carpeted house is an ideal breeding ground for fleas.