The rise of coronary heart disease and other chronic diseases led to greater recognition of the inapplicability of cause-and-effect models based on laboratory experiments involving bacterial diseases. A more suitable conceptual framework was correlation, a new method of finding associations rather than causes. Correlation was one of the most important new scientific concepts in the twentieth century and greatly expanded the methods that could be used to study the causes, prevention, and treatment of disease.

Bacteriology and the Doctrine of Specific Etiology

Prior to the nineteenth century, disease causation was a highly flexible concept that involved attributes of both the individual, such as age, gender, occupation, and heredity, and the environment, including geographic lo cation, climate, and characteristics of the population. The large number of causal factors and their many interrelationships made the numerous theories of disease etiology too complex to be useful in public health and clinical medicine.

The modern approach to disease etiology emerged in the early nineteenth century. Pathologists used autopsies to identify pathological lesions in the organs of deceased patients and related them to the patient's medical condition prior to death. This produced the principle that a precise causal relationship existed between changes in specific organs and the symptoms of a disease. The primary methodological limitation of the autopsy was that it occurred after death, making it impossible to demonstrate that the changes in the organs preceded the symptoms.

In the late nineteenth century bacteriology provided a major impetus to etiological models because it surmounted most of the methodological limitations of the autopsy and drew its findings from tuberculosis and other important infectious diseases. Bacteriological experiments using laboratory animals demonstrated that: (1) virulent bacteria could be removed from the bodies of diseased animals and grown in cultures, thereby proving that they existed independently of the animal and were not part of the animal's constitution; (2) pure bacteria from the cultures could be introduced into healthy animals, which developed the same disease promptly; (3) the disease never developed without the presence of the bacteria in the animal.