Similar changes occurred in other industrialized nations. In general, life expectancy increased for all groups, but significant differences were closely correlated with race and gender. In the United States, life expectancy was longer for women than men and longer for whites than nonwhites. American medical records, including reports of births, deaths, and specific diseases, however, are not very accurate for the period before the 1930s. In 1900 influenza and pneumonia, tuberculosis, and gastritis were the top killers, but by 1950 heart disease, cancer, and cerebrovascular disorders were the major causes of death.
Research on the nature of infectious diseases and their means of transmission led to the discovery and classification of many pathogenic organisms. In addition to bacteria, scientists identified the rickettsias, which cause such diseases as typhus, and pathogenic protozoans, such as those that cause malaria. Some infectious diseases were attributed to mysterious microbial agents that were invisible under the microscope and small enough to pass through filters that trapped bacteria. These entities were operationally defined as invisible, filterable viruses. Although viruses clearly were able to reproduce and multiply in plant and animal hosts, they could not be cultured in the laboratory. Early studies of viruses included Peyton Rous's demonstration in 1910 that a virus could transmit a malignant tumor in chickens.
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