96 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
has been claimed that syphilis may be thus transmitted by the male 
to the ovum in utero, the mother remaining uninfected by the disease. 
In prenatal infections the organisms must pass the placental barrier. 
This implies that the fetus becomes infected directly from the maternal 
blood stream, or by continuity of growth of the organisms through the 
placenta. The placental form of infection is not conceded by all 
observers, but it is reasonably certain that congenital syphilis may be 
contracted thus. Theobald Smith has found that Bacillus abortus^ 
and Vibrio fetus^ tend to localize in the bovine fetal membranes. 
Smallpox, measles, dysentery, various pyogenic infections, and, 
rarely, pneumonia are occasionally said to be prenatally transmitted 
to the fetus. With respect to tuberculosis, there is difference of 
opinion. A very few cases are on record in which prenatal infection 
seems almost certainly to have taken place, for the new-born infant 
exliibited lesions which were so far advanced that no other explanation 
than prenatal infection sufficed to explain them. 
C. Portal of Entry; Atria of Invasion.— The bacteria which cause 
infection in the human body may be provisionally divided into two 
great groups: those of exogenous origin, which are not habitual parasites 
of man; and those of endogenous origin, which are habitual parasites 
of man. 
The great majority of specific microbic diseases (in contradistinc- 
tion to non-specific inflammations) are incited by bacteria of exogen- 
ous origin. These organisms must enter the host directly through 
their respective appropriate atria to produce characteristic disease. 
For example, the typhoid bacillus only causes typhoid fever when 
the organism is swallowed and enters the body through the intestinal 
tract. Infection of a. skin wound with typhoid bacilli will not result 
in typhoid fever. Similarly, cholera vibrios do not produce the dis- 
ease cholera unless they enter the body through the gastro-intestinal 
tract, although if cholera vibrios are introduced through the skin in 
experimental animals they tend to migrate toward the intestinal tract, 
thus suggesting a special affinity for the intestinal tissues. Pathogenic 
bacteria of exogenous origin produce in general, progressive specific 
disease from man to man. Bacteria of endogenous origin, on the other 
hand— those which occur habitually as "opportunists" on the surface 
of the body or on mucous membranes opening to the exterior— ordina- 
rily exist as harmless parasites. They may, however, and occasionally 
do, become invasive, inciting local or generalized inflammatory reac- 
tions as a rule, rather than well-defined clinical syndromes which are 
frequently so characteristic of infections with exogenous pathogenic 
bacteria. The bacteria of the "opportunist" type do not ordinarily 
gain entrance to the tissues of the body through sharply-circumscribed 
atria and the disease they produce is usually not epidemic in character. 
1 Jour. Exper. Med., 1919, 29, 451. 
2 Ibid., 1919, 30, 313. 
