The infectious diseases which they produce in man, although among the most serious are less in number than those produced by bacteria.

So marked is the tendency to parasitism that they are often parasitic for each other, smaller forms entering into and living upon the larger.

Variation does not seem to be so marked in the protozoa as in the bacteria, though this is possibly due to our greater ignorance of them as a class.

We are not able, except in rare instances, to grow them in pure culture, and study innumerable generations under changes in the environment, as the bacteria have been studied.
If we regard the living things on earth from the narrow point of view as to whether they are necessary or useless or hostile to man, the protozoa must be regarded as about the least useful members of the biological society.

It is very possible that such a conclusion is due to ignorance; so closely are all living things united, so dependent is one form of cell activity upon other forms that it is impossible to foretell the result of the removal of a link.