The question is, therefore, reduced to the prime cause of the disease itself.
A theory that animalcules are the cause of the various contagious and infectious disorders has created much discussion; and although this opinion is not generally entertained by the faculty, the idea is so feasible, and so many rational arguments can be brought forward in its support, that I cannot help touching upon a topic so generally interesting.
In the first place, nearly all infectious diseases predominate in localities which are hot, damp, swampy, abounding in stagnant pools and excluded from a free circulation of air.

In a tropical country, a residence in such a situation would be certain death to a human being, but the same locality will be found to swarm with insects and reptiles of all classes.
Thus, what is inimical to human life is propitious to the insect tribe.
This is the first step in favor of the argument.

Therefore, whatever shall tend to increase the insect life must in an inverse ratio war with human existence.
When we examine a drop of impure water, and discover by the microscope the thousands of living beings which not only are invisible to the naked eye, but some of whom are barely discoverable even by the strongest magnifying power, it certainly leads to the inference, that if one drop of impure fluid contains countless atoms endowed with vitality, the same amount of impure air may be equally tenanted with its myriads of invisible inhabitants.
It is well known that different mixtures, which are at first pure and apparently free from all insect life, will, in the course of their fermentation and subsequent impurity, generate peculiar species of animalcules.

Thus all water and vegetable or animal matter, in a state of stagnation and decay, gives birth to insect life; likewise all substances of every denomination which are subjected to putrid fermentation.

Unclean sewers, filthy hovels, unswept streets, unwashed clothes, are therefore breeders of animalcules, many of which are perfectly visible without microscopic aid.
Now, if some are discernible by the naked eye, and others are detected in such varying sizes that some can only just be distinguished by the most powerful lens, is it not rational to conclude that the smallest discernible to human intelligence is but the medium of a countless race?
that millions of others still exist, which are too minute for any observation?
Observe the particular quarters of a city which suffers most severely during the prevalence of an epidemic, In all dirty, narrow streets, where the inhabitants are naturally of a low and uncleanly class, the cases will be tenfold.