We are now confronted with two questions:

1. In what manner does the tubercle-bacillus enter into the human
organism?

2. Under what conditions is the tubercle-bacillus able to generate
pulmonary consumption after it has entered the human organism?

All investigations, both of earlier and later date have established the
fact that the tubercle-bacillus is inhaled with the air, and then it is
mainly the foul air which is accused. But foul air is especially found
in such places where people congregate, as in rooms, barracks,
factories, etc. As it is a fact that there are always several
consumptives among a number of people, so in this case there will always
be occasion to inhale the tubercle-bacilli that have been cast out by
the consumptives. Therefore it is not the foul air in itself which
generates pulmonary consumption, but the circumstance that in this
connection there are always people present which are able to spread and
scatter the bacilli.

Luckily the physical qualities of the tubercle-bacilli are such that
they mostly adhere to the ground or floor and are rarely scattered in
the air as dust; otherwise pulmonary consumption would be much more
frequent than it is at present. Unfortunately the bacilli are very often
spread through uncleanliness of the people, because they touch objects
with their fingers to which the tubercle-bacilli chance to stick and
then they touch their mouth or nose with these fingers. In this way
bacilli can be taken into the system especially easily with the food.
Children are particularly exposed to contamination, crawling about on
the ground, on which, perhaps but recently, a consumptive has spit, and
more so because they often have the habit to put all sorts of things
and also the generally dirty fingers into their mouth.

On the other hand there are various obstacles in the way of
tubercle-bacilli entering the lungs. The distance from the mouth to the
lungs is long and narrow; all sorts of projections check the further
penetration of the bacilli. The trachea and the air-passages of the
lungs possess equipments arranged for the purpose of ejecting small
foreign substances, thus also to throw out the bacilli. In short it is
not too easy a matter for the bacilli to penetrate into the lungs.

And yet this happens only too often. For instance, in some people the
passage from the mouth down may be a wide one, so that the bacilli can
enter more easily; the protective arr