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Unlabelled: Salmonella groups of organisms are recognized as predominant in food poisoning outbreaks, but knowledge of the primary source of Salmonella bacilli and of paths of infection to implicated food is incomplete.-Unsolved problems discussed.-Food animals suffering from Salmonella infection are common in Germany but comparatively rare in this country and in most outbreaks the original food is shown to have been sound.-Infection of food from a human carrier is extremely rare. Detailed study of individual outbreaks brings out the striking fact that the Salmonella strains which cause food poisoning are just those types capable of causing disease in both man and animals. The hypothesis which best explains bacterial causation of most outbreaks is that the source of infection is derived, in most cases, from animals suffering from Salmonella disease or acting as carriers of these bacilli. Many facts favour this view, including the widespread extent of Salmonella infections in animals associated with food., Examples: Cows suffering from Salmonella infections with these bacilli in the milk; the widespread prevalence of such infections in rats and mice; the considerable extent to which pigs are infected with Salmonella bacilli.-Attention is directed to the presence of specific agglutinins in the blood of food animals such as bullocks and pigs, also to the fact that while in man a carrier condition for food-poisoning bacilli is extremely rare and at best a transient condition in animals. It is a well attested phenomenon. The causes of this difference are worthy of further study.Not only are there numerous types within the Salmonella group but these types exhibit characteristic and definite differences of pathological behaviour. Bacillus paratyphosus B and Bacillus aertrycke furnish a good illustration and these two types are critically contrasted.While our present knowledge is imperfect, it is suggested that the types in this group fall into three groups, i.e., (a) Strains pathogenic to man but not to animals; (b) strains pathogenic to animals but harmless to man; (c) strains pathogenic both to man and to animals. Only the last group cause food-poisoning outbreaks and represent the original and less specialized types.Varying virulence of Salmonella strains and problems thus raised in relation to food poisoning outbreak are discussed.