There are two types of diseases, first those which come by a disorder of functioning and the second which are the result of the invasion by outside agents. The outside agents are bacteria, or, as they are often called, germs, and the cold is a bacterial germ infection.
If a microscopical examination is made of the front part of the nose, it will be found, even in a healthy person, to contain numbers of bacteria of different varieties. Farther back in the interior of the nasal cavities, however, no bacteria will normally be found, owing to the cleansing and disinfecting power of the mucous lining. So we see that bacteria are always on the spot, but that in the ordinary way, that is in state of health, the nose can resist them. Why then, it may be asked, can we not sit back at our ease and leave everything to this protection mechanism?
To answer this we must imagine that two opposing force, the infecting agents or germs on the one hand and the protecting mechanism on the other, are in the balance against each other, and that a little weight added to or removed from either side of the scale may decide the issue between them.