Food sometimes serves as the medium for the introduction of parasites or their embryos, such as the tapeworm, roundworm, echi-nococcus, and trichina.

In many cases the source of infection is found in the consumption of raw or very imperfectly cooked swine flesh and other meats used in the manufacture of sausages and similar preparations which are carelessly handled and come in contact with the viscera of the animals in which the parasites or embryos reside in some intermediate stage of development. Another possible source of infection, especially of intestinal worm larvae, is in the excrement of animals, which is permitted to pollute raw vegetables growing upon the ground. Manure is freely spread about, and dogs infested with worms, or a casual pig allowed at large in a vegetable garden, may contaminate by their faeces such vegetables as grow low upon the ground and are usually eaten raw, as lettuce, celery, cabbage, etc. Fortunately, this is by no means a common source of infection, but it is well to remember that several diseases have been definitely traced to such a source, and this explains the occasional presence of tapeworms in people who never eat raw meats, ham, or sausage, or the flesh of swine in any form. No animal should ever be allowed to roam in a vegetable garden.

Other varieties of parasites or their larvae may occasionally be introduced with the food or water, such, for example, as the Dracunculus medinensis, which produces the Guinea-worm disease, or Dracontiasis, the larvae of which are sometimes swallowed in drinking water by the natives of parts of Africa and the East Indies.