216 ATOMIC MEDICINE irradiated control animals. With virulent organisms, an appreciable inerease in mortality rate is seen at x-radiation dose levels below those producing mort ality in the absence of challenge (Shechmeister et al.). With normally nonvirulent organisms, the mortality rate following exposure in the lethal range is appreciably enhanced. Increased susceptability to viruses, Rickettsia, parasites such ax T'richinella spiralis, and bacterial toxins in the ir- radiated animal have been reported. A synergistic effect of x-radiation and cortisone in increasing susceptibility to administered bacteria and viruses has been reported (Friedmanet ai.). , It is of importance to point out, however, that there is considerable variation in the degree of increased sensitivity to different microorganisms or toxins, and in the degree of effects depending on the route of administration (see under mechanisms of increased susceptibility below). Specifically, Hale and Stoner have shown that although a markedincrease in susceptibility to pneumococci ix evident in the irradiated mouse, no such increase is found for mice challenged with a virulent influenza virus. With the virus innoculation, the animals were protected against secondarybacterial invasion with antibiotics. The degree to which the reported increased susceptibility to viruses in irradiated individuals is due to secondary bacterial invasion is not known. Neutrophiles play a large role in resistance against bacterial, but not viral diseases (Wood). The peripheral neutrophile count is profoundly affected by irradiation, and thus a greater degree of altered immunity to bacterial, rather than viral infections might be expected. At any rate, blanket statements relating to susceptibility of the irradiated host are not warranted, and specific consideration in each instance must be given to the infecting agent, the host, the physiological state of the host and the degree of exposure to both radiation and the infectious agent. Germ-free animals die following total body exposure to x-radiation (Reyniers). The dose required to kill, however, is somewhat higher than for “normal” animals, and the survival time is increased. These results can be taken as indicating that infection may be responsible for death in the irradiated animals in certain dose ranges. At higher doses, animals die even in the absence of infection. Extensive hemorrhage probablyis a major eause of death in such animals; however, undoubtedly other poorly understood biochemical changes contribute. Antibiotics administered following radiation exposure have been shown definitely to enhance survival under some circumstances. The incidence of spontaneous mortality has been shown to be reduced, and mortality in x-irradiated mice subsequently challenged with virulent organisms has been decreased. Antibiotics, however, should not be considered to be necessarily lifesaving following severe exposure in the human being. Two human beings