entrance we can guard the skin with the greatest difficulty.  We can easily render food free from pathogenic bacteria by heating it.  The material we drink can similarly be rendered harmless, but we can not by any known means avoid breathing air, nor is there any known method of disinfecting the air, and it is impossible for those who have anything to do with sick persons to avoid entirely having contact either with the patient or with infected clothing or utensils.

From the facts here given it will be seen that the individual’s susceptibility to disease produced by parasitic bacteria will depend upon his habits of cleanliness, his care in handling infectious material, or care in cleansing the hands after such handling, upon his habit of eating food cooked or raw, and upon the condition of his skin and mucous membranes, since any kind of bruises will increase susceptibility.  Slight ailments, such as colds, which inflame the mucous membrane, will decrease its resisting power and render the individual more susceptible to the entrance of any pathogenic germs should they happen to be present.  Sores in the mouth or decayed teeth may in the same way be prominent factors in the individual’s susceptibility.  Thus quite a number of purely physical factors may contribute to an individual’s susceptibility.

Resisting Power of the Body.—­Even after the bacteria get into the body it is by no means certain that they will give rise to disease, for they have now a battle to fight before they can be sure of holding their own.  It is now, indeed, that the actual conflict between the powers of the body and these microscopic invaders begins.  After they have found entrance into the body the bacteria have arrayed against them strong resisting forces of the human organism, endeavouring to destroy and expel them.  Many of them are rapidly killed, and sometimes they are all destroyed without being able to gain a foothold.  In such cases, of course, no trouble results.  In other cases the body fails to overcome the powers of the invaders and they eventually multiply rapidly.  In this struggle the success of the invaders is not necessarily a matter of numbers.  They are simply struggling to gain a position in the body, where they can feed and grow.  A few individuals may be entirely sufficient to seize such a foothold, and then these by multiplying may soon become indefinitely numerous.  To protect itself, therefore, the human body must destroy every individual bacterium, or at least render them all incapable of growth.  Their marvellous reproductive powers give the bacteria an advantage in the battle.  On the other hand, it takes time even for these rapidly multiplying beings to become sufficiently numerous to do injury.  There is thus an interval after their penetration into the body when these invaders are weak in numbers.  During this interval—­the period of incubation—­the body may organize a resistance sufficient to expel them.

We do not as yet thoroughly understand the forces which the human organism is able to array against these invading foes.  Some of its methods of defence are, however, already intelligible to us, and we know enough, at all events, to give us an idea of the intensity of the conflict that is going on, and of the vigorous and powerful forces which the human organism is able to bring against its invading enemies.