_Epidemiology._--The conditions under which _B. botulinus_ occurs and is given opportunities for multiplying are not completely known. It is possible that there are localities where this bacillus is particularly abundant in the soil or in the intestinal contents of swine or other domestic animals, but on the whole it seems more probable that the organism is widely distributed, but that it does not often find suitable conditions for entrance into, and multiplication in, human food. Practically all the reported cases of botulism have been caused by food which has been given some sort of preliminary treatment, as smoking, pickling, or canning, then allowed to stand for a time, and _eaten before cooking_. Since both the bacillus, including the spore stage, and its toxin are destroyed by relatively slight heating, it is clear that a rather unusual set of factors must co-operate in order that botulism poisoning shall take place. These are evidently: (1) the presence of the bacilli in sufficient numbers in a suitable foodstuff; (2) the initial preparation of the food by a method that does not destroy the _B. botulinus_--inadequate smoking, too weak brine,[109] or insufficient cooking; (3) the holding of this inadequately preserved food for a sufficient length of time under the right conditions of temperature and lack of oxygen; (4) the use of this food, in which conditions have conspired to favor the production of toxin by _B. botulinus_, without final adequate cooking. It seems as reasonable to suppose that the infrequency with which these several factors coincide is responsible for the relative uncommonness of botulism as to suppose it due to the rarity of the specific bacillus. In the Belgian outbreak studied by Van Ermengem the poisonous ham had lain at the bottom of a cask of brine (anaërobic conditions) while the other ham of the same animal lay on top of it but was not covered with brine, and was eaten without producing any poisonous effect. In this instance the presence or absence of favorable conditions for anaërobic growth seemed to be the decisive factor.