The common =domestic fly= (_Musca domestica_) is the unwelcome companion of man in nearly every country. The eggs are usually laid and the larvæ undergo their development in excrement, but the female sometimes selects meal, bread, or fruit for the purpose. In practice, however, one of the best means of diminishing the number of domestic flies is to insist on the daily removal of all manure, especially horse manure, and to sprinkle the manure receptacle in the interval with lime. The fly may obviously be the means of conveying infected material from place to place. Anthrax has been ascribed to this cause. Nuttall has proved experimentally that flies are able to carry the infection of plague, and that they die of the disease. The presence of enormous numbers of flies in cholera times has been noted. Experimentally, flies caught in cholera wards have been found to harbour the cholera spirillum. It is probable that they play a serious rôle in spreading the infection of cholera. Hence all infectious dejecta (stools and urine) should be covered until finally disposed of, and food should be protected against flies. The same remarks apply for enteric fever. Flies fed with pure cultures of the bacillus of enteric fever pass these bacilli in their dejecta in a still virulent condition. In camps, especially in connection with large armies, there is the strongest reason for believing that flies carry infection from latrines to food. Flies have been known to feed on the expectoration of consumptive patients and it is possible therefore that they may thus infect food. The =bed bug= (_Cimex lectularius_) has been stated to be capable of conveying by its bite the infection of plague and other diseases from an infected to a healthy person; but Nuttall’s experimental results were entirely negative. =Fleas= (_pulex_) probably do not play any part in spreading anthrax. Experimentally, anthrax bacilli die off rapidly in fleas. In India, persons who had handled rats dead of plague frequently acquired the disease. This was explained by Simond on the supposition that the fleas abandoned the dead rat for the human subject. The rats which appeared to have caused plague in man were stated to have died but a short time before; and the handling on the day after their death of rats dead of plague was stated to be safe because the rats’ fleas had then deserted the dead rat. It is assumed that the flea injects the poison of plague under the skin. On the contrary it is to be remembered that the fleas infesting rats and mice belong to a different family from that which attacks man. Whether this is a usual means of conveying plague may therefore be regarded as still doubtful. That rats convey plague to man is certain; whether fleas act as an intermediary remains somewhat uncertain.