 In many species, fleas are principally a nuisance to their hosts, causing an itching sensation which in turn causes the host to try to remove the pest by biting, pecking or scratching.Fleas are not simply a source of annoyance, however.Flea bites cause a slightly raised, swollen, irritating nodule to form on the epidermis at the site of each bite, with a single puncture point at the centre, like a mosquito bite.