Although we know it is due to a germ, we don't yet know exactly how that germ is conveyed from one victim to another.

One thing, however, of great practical importance we do know, and that is that pneumonia is a disease of overcrowding and foul air, like tuberculosis; that it occurs most frequently at that time of the year--late winter and early spring--when people have been longest crowded together in houses and tenements; and that it falls most severely upon those who are weakened by overcrowding, under-feeding, or the excessive use of alcohol.

How strikingly this is true may be seen from the fact that, while the death-rate of the disease among the rich and those in comfortable circumstances, who are well-fed and live in good houses, is only about five per cent,--that is, one in twenty,--among the poor, especially in the crowded districts of our large cities, the death-rate rises to twenty per cent, or one in five; while among the tramp and roustabout classes, who have used alcohol freely, and among chronic alcoholics, it reaches forty per cent.

The same steps should be taken to prevent its spread as in tuberculosis--destroying the sputum, keeping the patient by himself, and thoroughly ventilating and airing all rooms.
As the disease runs a very rapid course, usually lasting only from one to three weeks, this is a comparatively easy thing to do.
Though pneumonia is commonly believed to be due to exposure to cold or wet, like colds, it has very little to do with these.

You will not catch pneumonia after breaking through the ice or getting lost in the snow, unless you already have the germs of the disease in your mouth and throat, and your constitution has already been run down by bad air, under-feeding, overwork, or dissipation.