A number of diseases have been found to produce acute, temporary brain disorders as a result of systemic infections that do not invade the central nervous system. The most important are pneumonia, diphtheria, typhoid fever, uremia, pernicious anemia, influenza, malaria, rheumatic fever, smallpox, scarlet fever, and un- dulant fever. Brain disorders associated with intracranial infection are discussed under meningitis, sydenham’s chorea, EPIDEMIC ENCEPHALITIS, MENINGOVASCULAR SYPHILIS, GENERAL PARESIS, and JUVENILE PARESIS.The dominant symptom in these cases is a mild to intense delirium. It occasionally appears during the incubation, or pre-febrile, period, but is most acute during the febrile stage and may continue in the post-febrile period. In some instances, however, it sets in after the temperature has dropped to normal. In cases where the patient has suffered prostration and exhaustion, it may take a particularly severe and dangerous form known as “collapse delirium.”The intensity of the delirious state has been found to depend more on the life situation and personality of the patient than on the height of the fever. Well-integrated individuals do not usually develop mental symptoms even in the most severe illnesses, but even a mild fever may bring on delirium in the poorly adjusted. If the delirium runs its full course it usually shows fairly definite stages. The early symptoms are restlessness, sensitivity to noise and light, and disturbing dreams. This phase may be followed by clouding of consciousness, disorientation for time and later for place and person, impairment of concentration, attention, and understanding. Visual illusions, hallucinations and transient delusions may then set in. These are usually accompanied by apprehensions and fears which may be traceable to the patient’s actual worries or repressed urges. If the delirium progresses to the acute stage, the patient becomes increasingly confused and agitated, and may be subject to periods of drowsiness and coma.Treatment is primarily directed at the infection, but the delirium itself can often be controlled by ice packs, continuous baths or packs, and tranquilizing drugs, accompanied by such psychological measures as reassurance, a quiet and unstimulating environment, and the presence of the patient’s family. Disorientation for time, place, and person clears up before the hallucinations disappear, and ordinarily the patient returns to normal a short time after the fever subsides. There is no damage to the brain except in rare cases where the illness has been both severe and prolonged.