Epidemiology is the study of epidemics. The primary goal of epidemiological studies should be the identification of the determinants of disease in order to decrease morbidity and mortality. Epidemiological studies evolve through descriptive, analytical, and experimental approaches. The traditional infectious disease epidemiology studies were primarily concerned with identification of an agent, incubation period, mode of transmission, population at risk, and methods of disease control. Chronic disease epidemiology has tended to emphasize a more complex interaction of independent and dependent disease variables that resulted in a greater need for statistical methodology. There has been relatively little interest in chronic disease epidemiology either in modes of disease transmission or in incubation periods. Chronic disease epidemiology has also focused more on analytical epidemiology than on experimental, clinical trials. Many chronic diseases are probably caused by living organisms such as viruses. The fundamental difference in methodology may relate to length of incubation period. Chronic disease epidemiology should probably build more on successful methods of infectious disease epidemiology, especially modes of disease transmission, host susceptibility, incubation periods, and clinical trials. The concept of multifactorial etiology of many chronic diseases may be a measure of our ignorance of causality rather than a biological principle.