mentioned in that the organism which causes the disease must live for a time in the body of some other animal to undergo certain definite changes before it can again induce the disease in another individual. The most important insect-borne diseases belong to this type, for in the case of man and domestic animals, certain insects and ticks act as the secondary host animals for the organisms of many diseases. Thus, yellow fever is spread only through the agency of a certain mosquito, for in its body alone can the yellow fever organism live and undergo the changes that are necessary before it can be introduced into another patient by the bite of an infected mosquito. Malaria belongs to the same category, for it spreads only through the bite of certain mosquitoes that obtain the organisms with their meal of blood, and then afterwards inject into the blood of another person, a later stage of the malarial parasite which has developed meanwhile within the mosquito.
Diseases in which certain insects act as specific carriers are most numerous and prevalent in the warmer parts of the world, although temperate regions are by no means free from diseases of this kind, which may be referred to as the inoculative type.
Among insects which disseminate pathogenic organisms without any specific association, the common housefly is without doubt the most important. The rank of the fly in this unenviable profession is due to several facts in its life-history which render it eminently suited to act as a vector for several diseases such as typhoid fever, diarrhœa, dysentery, summer complaint of children, etc. The eggs of the house fly are laid preferably in horse manure, upon which the larval stages or maggots feed, but human excrement serves equally well, and when exposed is very likely to provide food for a brood of fly maggots. Less than two weeks are required for the larvæ to mature, and after a short resting stage of two or three days the adult flies emerge from their bed of filth. At this time they may quite possibly bear in their alimentary tract bacteria derived from their larval food. Usually, however, they are quite clean when hatched, in spite of the surroundings whence they have come. They do not long remain so, however, for they feed upon animal and human dejecta of all sorts, garbage and other fermenting material, and if still hungry invade markets or houses, where they may leave upon food any bacteria brought upon their bodies, legs, mouth parts or wings. In addition, their excrement deposited as "fly specks" may contain virulent pathogenic bacteria, if they have had access to matter from which these could be obtained.
This, in brief, is the status of the house fly as a disease carrier and it is readily seen why filth diseases are the ones naturally spread by this insect. Attracted to the nursing bottle or to the baby's mouth they may infect him with bacteria of any of the numerous enteric troubles known as summer complaint. If they have had access to the dejecta of a person