The nagana, or tsetse-fly disease of cattle is the most virulent disease of domestic animals in certain parts of Africa. It is caused by a protozoan blood parasite, _Trypanosoma brucei_, which is conveyed to healthy animals by the bite of _Glossina morsitans_ and possibly other species of tsetse-flies. The flies remain infective for forty-eight hours after feeding on a diseased animal. The insect also serves as an essential host of the parasite. Surra, a similar trypanosomiasis affecting especially horses and mules, occurs in southern Asia, Malaysia, and the Philippines where the tsetse-flies are not to be found. It is thought to be spread by various species of blood-sucking flies belonging to the genera _Stomoxys_, _Hæmatobia_, and _Tabanus_. Mitzmain (1913) demonstrated that in the Philippines it is conveyed mechanically by _Tabanus striatus_. The sleeping sickness of man, in Africa, has also been supposed to be directly inoculated by one, or several, species of tsetse-flies. It is now known that the fly may convey the disease for a short time after feeding, but that there is then a latent period of from fourteen to twenty-one days, after which it again becomes infectious. This indicates that in the meantime the parasite has been undergoing some phase of its life-cycle and that the fly serves as an intermediate host. We shall therefore consider it more fully under that grouping. These are a few of the cases of direct inoculation which may be cited as of the simpler type. We shall next consider the rôle of the flea in the dissemination of the bubonic plague, an illustration complicated by the fact that the bacillus multiples within the insect and may be indirectly inoculated. THE RÔLE OF FLEAS IN THE TRANSMISSION OF THE PLAGUE The plague is a specific infectious disease caused by _Bacillus pestis_. It occurs in several forms, of which the bubonic and the pneumonic are the most common. According to Wyman, 80 per cent of the human cases are of the bubonic type. It is a disease which, under the name of oriental plague, the pest, or the black death, has ravaged almost from time immemorial the countries of Africa, Asia, and Europe. The record of its ravages are almost beyond belief. In 542 A. D. it caused in one day ten thousand deaths in Constantinople. In the 14th century it was introduced from the East and prevailed throughout Armenia, Asia Minor, Egypt and Northern Africa and Europe. Hecker estimates that one-fourth of the population of Europe, or twenty-five million persons, died in the epidemic of that century. From then until the 17th century it was almost constantly present in Europe, the great plague of London, in 1665 killing 68,596 out of a population of 460,000. Such an epidemic would mean for New York City a proportionate loss of over 600,000 in a single year. It is little wonder that in the face of such an appalling disaster suspicion and credulity were rife and the wildest demoralization ensued.