"Just as a certain plant comes from the seed of the same plant and not from any plant at will, so each contagious disease must be propagated from a similar disease and cannot be the result of any other disease." Further he says, "It is necessary to assume that during the prevalence of an epidemic the contagious material undergoes an enormous increase, and this is compatible only with the assumption that it is a living substance." But as is so often the case, speculation ran far ahead of the observations on which it is based.

There was a long gap between the observations of Loewenhoeck and the theories of Plenciz, justified as these have been by present knowledge.

In the spirit of speculation which was dominant in Europe and particularly in Germany in the latter half of the eighteenth and the first half of the nineteenth centuries, hypotheses did not stimulate research, but led to further speculations.

As late as 1820 Ozanam expressed himself as follows: "Many authors have written concerning the animal nature of the contagion of disease; many have assumed it to be developed from animal substance, and that it is itself animal and possesses the property of life.

I shall not waste time in refuting these absurd hypotheses." The theory of a living contagion was too simple, and not sufficiently related to the problems of the universe to serve the medical philosophers.
Knowledge of the minute organisms was slowly accumulating.