_History._--This disease has been known in England and continental Europe for many years, and descriptions of it are mentioned in the writings of Mascal, Lafoose, Skellet, Lawrence, St. Cyr, Z¸ndel, and Youatt. In the early part of the eighteenth century British veterinarians recognized its contagiousness, but it remained for Franck (1876), Lehnert (1878), and Br‰uer (1880) to produce the disease in healthy, pregnant cows by the introduction of exudate and material from aborting animals. Nocard (1888) isolated from the exudate between the mucous membrane of the uterus and fetal membranes a micrococcus and a short bacillus which were found continually in contagious abortion, but he failed to reproduce the disease by inoculations of pure cultures of these organisms into healthy, pregnant animals. In 1897 Bang, assisted by Stribolt, published their findings regarding infectious abortion of cattle, in which they incriminated Bang's bacillus of abortion as the causative agent. With pure cultures of this bacillus they were able to produce the disease artificially and to recover the same organism from the experimental cases. Since that time many noted investigators, both in this country and in Europe, have confirmed these findings. _Cause._--The _Bacterium abortus_ of Bang is now generally recognized as the causative agent of the disease of cattle. Formerly it was thought that abortion was due to injury, such as blows, horn thrusts, falls, etc., or the eating of spoiled feed and certain plants, and while this may be true in a limited number of cases, careful investigations have demonstrated these claims to be largely unfounded. It is now generally recognized that when abortion occurs in herds from time to time, it is safe to assume that the disorder is of an infectious nature and should be so treated. _Natural mode of infection._--This phase of the disease is of greatest importance for a clear understanding of the methods of prevention. Many investigators have demonstrated that the infection is transmitted through the digestive tract, through contaminated feed and water. The germs are taken up by the body from the intestines with the liquid nourishment, reach the blood, and are carried to the genital organs, where they find conditions best suited to their development. Some assert that calves are infected in this manner by suckling infected mothers, the germs being present in the milk, or the teats having been contaminated by coming in contact with infective discharges. It is claimed that infection contracted in this manner remains dormant in the body of the calf until pregnancy begins, and then the organism, finding conditions suitable for its development, produces the disease.