Metaphor for the Spread of Disease

To choose a rough example, think of a thorn which has stuck in a finger and produces an inflammation and suppuration. Should the thorn be discharged with the pus, then the finger of another individual may be pricked with it, and the disease may be produced a second time. In this case it would not be the disease, not even its product, that would be transmitted by the thorn, but rather the stimulus which engendered it. Now supposing that the thorn is capable of multiplying in the sick body, or that every smallest part may again become a thorn, then one would be able to excite the same disease, inflammation and suppuration, in other individuals by transmitting any of its smallest parts. The disease is not the parasite but the thorn. Diseases resemble one another, because their causes resemble each other. The contagion in our sense is therefore not the germ or seed of the disease, but rather the cause of the disease. For example, the egg of a taenia is not the product of a worm disease even though the worm disease may have been the cause, which first gave rise to the taenia in the intestinal contents—nor of the individual afflicted with the worm disease, but rather of the parasitic body, which, no matter how it may have come into the world at first, now reproduces itself by means of eggs, and produces the symptoms of the worm disease, at least in part. It is not the seed of the disease; the latter multiplies in the sick organism, and is again excreted at the end of the disease.

Like spreading thorns.