Whilst white men have developed an almost automatic resistance and objection to the visits of flies to their lips, eyelids, and any wound or scratch of the skin--a resistance which is not shown by many savage races--they yet allow house-flies to swarm in their dwellings, to run about and sample their food, with an indifference which is, when the truth is known, truly horrible in its fatuity and foolhardiness. For the fact is that the feet and proboscis of the common house-fly are covered with microbes of all sorts, picked up by his explorations upon every kind of filth. At every step which he takes he plants a few dozen microbes, which include those of infantile diarrhoea, typhoid, and other prevalent diseases. This is easily shown by allowing him to walk over a smooth plate of sterilised nutritive gelatine and preserving it afterwards free from the access of microbes from the air. In twenty-four hours every footstep of the fly on the gelatine is marked by an abundant and varied crop of microbes, which have multiplied from the individuals let drop by the little pedestrian. There is no doubt whatever that the house-fly is a main source of the dissemination of the microbe of infantile diarrhoea, and the cause annually of hundreds of thousands of deaths of children in the great cities of Europe and America. Also in camps and infected districts he is largely responsible for the introduction of the microbe of typhoid fever into the human food to which he has free access after his previous visits to open latrines. The house-fly is himself a product of dirt and neglect. The eggs are laid in old manure heaps and kitchen middens, and the maggots, which eventually are transformed into flies, nourish themselves in those accumulations. When this refuse is rapidly and regularly removed by the care of the sanitary officials of a town, the flies diminish in number, as they have diminished in London within the last thirty years. We no longer are overrun by flies in London in the summer months. The man selling sheets of sticky paper is no longer heard in our streets calling "Catch 'em alive, oh!" But in country places, where a neglected stable-yard is near the dining room of the inn, house-flies are as great a nuisance and danger as ever. There is no difficulty, if the simplest rules of cleanliness are observed, in abolishing them altogether from human association, but combined and simultaneous action against them is an essential condition of success. CHAPTER XXII IMMUNITY AND CURATIVE INOCULATIONS