The infective agent in the case of plague is the _bacillus pestis_, identified in 1894 by Kitasato, and subsequently, but independently, by Yersin. Rats, which are very susceptible to the disease, play an important rôle in spreading it; in India the outbreak of a plague epidemic is always preceded by the dying of large numbers of rats. Their excrement contains large quantities of bacilli, which may be destructive to human beings. The rat-flea is also known to carry the infection. The infection may be conveyed directly by plague patients, when the buboes suppurate, or when the blood becomes generally infected with the _bacilli pestis_, which are contained in abundance in the sputum, urine, and excrement, or when the lungs are affected and the patient charges the atmosphere by coughing. One who has recovered from the disease is usually immune for life. 3. _Cholera_, after an incubation period of two to eight days, begins with frequent (ten to twenty times a day) vomitings of a fluid like rice-water, and incessant retching. The patient, owing to the great loss of water, sinks rapidly; he acquires a corpse-like appearance, loses consciousness, and death may result on the first or second day. If the attack is survived, the patient frequently dies from sheer exhaustion afterwards. The mortality of cholera is great—from forty to fifty per cent of its victims die. In this calculation the numerous cases of cholerine, that are always prevalent during cholera times, are excluded. Recovery from the disease does not protect a person against contracting it again. The infective germ in the case of cholera is the ‘comma bacillus’, discovered by Robert Koch in 1883. The spread of cholera is caused by the penetration of the _comma bacillus_ into the alimentary canal, resulting from contact with objects which have been contaminated by the evacuations of cholera patients; less frequently it is indirectly caused by the pollution, from evacuations, of water used for drinking or washing purposes. 4. _Dysentery_ has always played an important rôle in military campaigns. To be sure, it is not very dangerous, so far as the patient’s life is concerned, but in war times, owing to irregular nursing and scanty nourishment, and the consequent use of unsuitable food, it may spread over a large territory and be very destructive to large numbers of soldiers and other people.