TUBERCLE BACILLUS 475 
Trauma, establishmfj a locus luinoris resistentise to which tubercle 
bacilli lying dormant in the body may l)e transported and set up infec- 
tion, is pr()bal)ly uncommon. 
Conditions Favoring Infection.— Overcrowding with its attendant 
evils of dark, damp rooms, poor food and general unhygienic condi- 
tions appears to be a most potent factor in the spread of tuberculosis. 
No age is exempt, although the disease is somewhat less frequent 
between the ages of five and ten years, greatest between sixteen and 
thirty-five years. The sexes are about equally infected. Negroes 
are especially prone to the disease, possibly because of their surround- 
ings and manner of living rather than any inherent lack of resistance. 
Tuberculosis is relatively uncommon among the aboriginal negro 
races in Africa. Jews appear to be relatively immune to the disease. 
Those occupations in which dust is generated in large amounts exhibit 
a higher incidence of the disease than occupations in which dust is 
not a feature. Catarrhal infections of the respiratory tract appear 
to predispose to pulmonary tuberculosis as do measles, whooping 
cough aufl influenza, or to reacti^•ate latent infection with the tubercle 
bacillus. 
Atria of Invasion.— The respiratory and digestive tracts (including 
the tonsils) and the skin are the three portals through which tubercle 
bacilli enter the tissues of the body. Of these the respiratory tract 
is more frequently involved. Droplet infection is by far the most 
common method of transmission of tubercle bacilli; dust-borne infec- 
tion is probably relatively uncommon. 
Tubercle bacilli also enter the intestinal tract and they may pass 
through the intestinal mucosa without leaving any trace of their 
passage, particularly if they be suspended in fatty menstrua, as butter 
or cream. 1 The bovine type of the tubercle bacillus may enter through 
the tonsils, or the digestive tract occasionally. Rarely, tubercle bacilli 
enter through the skin, usually causing somewhat localized epidermal 
proliferations containing tubercle bacilli in small numbers, which 
are sometimes called butcher's warts, postmortem warts or verruca 
necrogenica. Usually they remain localized. Tuberculosis of the 
skin, or lupus, on the other hand, is not uncommon. 
For some years much discussion has centered upon the incidence of 
bovine tubercle bacillus infection in man. Koch was inclined to the 
view that infection with this organism was so rare as to be practically 
negligible. Later he modified his opinion. Weber'- studied ()28 cases 
(284 children, 335 adults, 9— age unstated), all of whom had drunk 
the milk of cows having tuberculosis of the udder, or had consumed 
uncooked products made from the milk. Only 2 patients, both 
very young children, were definitely shown to be infected with bovine 
tubercle bacilli. Both had enlarged caseous cervical glands from which 
1 Nicolas and Descos: Jour, physiol. et path, gen., 1902, 4, 910. Ravenel: Jour. 
Med. Res., 190.3. 10, 460. 
2 Tuberkulose, Arbeiten a. d. kais. Gesamte, 1910, Heft 10, 1. 
