In all foul air there are scores of different kinds of germs--many of them comparatively harmless, like the yeasts, the moulds, the germs that sour milk, and the bacteria that cause dead plants and animals to decay.

But among them there are a dozen or more kinds which have gained the power of living in, and attacking, the human body.

In so doing, they usually produce disease, and hence are known as _disease germs_.
[Illustration: DISEASE GERMS (Greatly magnified) (1) Bacilli of tuberculosis; (2) Bacilli of typhoid fever.] These germs--most of which are known, according to their shape, as _bacilli_ ("rod-shaped" organisms), or as _cocci_ (round, or "berry-shaped" organisms)--are so tiny that a thousand of them would have to be rolled together in a ball to make a speck visible to the naked eye.

But they have some little weight, after all, and seldom float around in the air, so to speak, of their own accord, but only where currents of air are kept stirred up and moving, without much opportunity to escape, and especially where there is a good deal of dust floating, to the tiny particles of which they seem to cling and be borne about like thistle-down.

This is one reason why dusty air has always been regarded as so unwholesome, and why a very high death rate from consumption, and other diseases of the lungs, is found among those who work at trades and occupations in which a great deal of dust is constantly driven into the air, such as knife-grinders, stone-masons, and printers, and workers in cotton and woolen mills, shoddy mills, carpet factories, etc.
[Illustration: A VACUUM CLEANER Most of the dust being emptied from the bag, would, in ordinary sweeping, have been merely blown around the room.