Any of the acute infections, as typhoid, typhus, smallpox, measles, scarlatina, and the others, attacks a pregnant woman as readily as one who is not pregnant. Pregnancy, as a rule, lessens the resistance to the infection, and the infection is likely to cause abortion. The toxin of the infection is added to the physiological toxins of pregnancy, the kidneys often are overwhelmed, and there is a tendency to hemorrhage. After the exhaustion from the disease, delivery, whether premature or at term, is liable to end in collapse, especially if the heart or lungs have been injured. Puerperal sepsis, either general or local, is a common effect of these bacterial diseases. In smallpox there is infection from the pustules and the virus itself; in typhoid the typhoid bacillus and the streptococci in Peyer's patches get into the blood; in influenza, pneumonia, erysipelas, and diphtheria the bacteria directly cause sepsis, and in scarlatina the pus organisms from the throat are found in the septic foci. In these infections the fetus may be killed by the high temperature; it may die from asphyxia brought on by feeble maternal blood-pressure and consequent stagnation of the circulation in the uterine sinuses; it may be overwhelmed by maternal hemorrhage; by deoxidation of the maternal blood, as in pneumonia; by a hemorrhage in the placenta, and a consequent separation of the placenta itself from the uterine wall; by fatty degeneration of the fetal villi, which renders respiration of gases impossible. Again, the child may be infected by the disease of the mother, or it may be killed by the toxins in the maternal circulation. The communication between the fetal and the maternal blood systems is as indirect as that between the air in a man's lungs and his blood. The communication between mother and fetus is by osmosis, but certain toxins, drugs, and bacteria may also pass from the maternal to the fetal circulation through the placenta. Strychnia injected directly into the embryos of animals by Savory and Gussarow killed the mother after passing to her through the placenta. There is no direct communication (except by osmosis) between the fetal chorionic villi and the maternal intervillous blood spaces. In the first half of pregnancy fetal and maternal blood are separated by the syncytium, Langhan's layer of cells, the stroma of the villi, and the walls of the fetal capillaries; in the second half of gestation Langhan's layer gradually disappears. In the fetal blood-vessels are found many nucleated red corpuscles, but these are lacking in the maternal intervillous spaces. Sänger also discovered that in pernicious leucemia the leucocytes of the mother are not present in the fetal circulation.