Until recently infection was believed to be caused solely by living organisms and their toxins, but there is some evidence in the case of viruses which makes it likely that some are nonliving. If this be so, the term "infectious agent" would be more nearly correct than the term "organism" when speaking of parasites. Both terms will be used, with the realization that some infections may be caused by nonliving entities. Briefly, the agents which cause infections in man may be classified into four groups: (1) plantlike organisms, (2) organisms of animal origin, (3) the rickettsieae, and (4) ultramicroscopic agents called viruses.

Plantlike organisms include yeasts, molds, and bacteria. Infections due to yeasts and molds are slightly communicable, as, for instance, thrush in infants or ringworm in children and adults. Bacteria include a formidable number of round organisms called cocci, rod-like organisms called bacilli, and spiral-like organisms called spirilla. Important disease-producing cocci are the following: streptococci, staphylococci, pneumococci, meningococci, and gonococci. Bacilli which produce disease in man include organisms causing typhoid and paratyphoid fever, whooping cough, diphtheria, tuberculosis, leprosy, undulant fever, tularemia, tetanus. dysentery, and plague, among others. Numbered among the spirilla are the spirochetes', which cause syphitis, yaws, and Weil's disease, and the organism which, in conjunction with a fusiform bacillus, causes Vincent's infection.

The animal kingdom is responsible for infection or infestation due to protozoa, the principal ones that affect man causing amebic dysentery and malaria; other infections of animal origin are actually infestations caused by worms. Viruses are responsible for many of the common communicable diseases: measles, rubella, smallpox, chickenpox, mumps, the enterovirus group of infections (Coxsackie, ECHO, and poliomyelitis), and influenza. Less common diseases caused by these agents are the arborvirus encephalitides, lym phocytic choriomeningitis, and rabies.

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