Certain of the infectious diseases are, as has been said, essentially toxic in their nature, and in cultures the organisms produce poisonous substances.

By the injection into the tissues of such substances the same disturbances are produced as when the bacteria are injected.

Such a disease is diphtheria.

In this there is only a superficial invasion of the tissues.

The diphtheria bacilli are located on the surface of the tonsils or pharynx or windpipe, where, as a result of their action, the membrane so characteristic of the disease is produced.