The complex and controversial history of this centuries-old public health strategy offers guidance for its future use.

The devastating influenza pandemic known as 'Spanish flu', which killed at least 20 million people all over the world in 1918, was responsible for the first bitter blow inflicted on triumphant bacteriology, fortified by the series of resounding successes achieved in identifying the pathogenic agents of terrible diseases such as anthrax, cholera, tuberculosis, plague, and syphilis. Over-confidence and the idea, born of the Pasteur revolution, that every infectious disease was caused by a bacterium, had led the scientific community to accept the theory put forward by the German bacteriologist, Richard Pfeiffer, who, in 1892, believed he had identified the pathogenic influenza agent in a bacterium, Haemophilus influenzae. But, while the most appalling epidemic ever to sweep through the world since the 'Black Death' of the 1300s was still raging, the scientific community had to admit that influenza originated not from a microbe, but from a virus. This article aims to reconstruct the enlightening and little-known cultured/scientific events and issues of the dramatic crisis that bacteriology experienced in the autumn of 1918, with the consequent simultaneous collapse of both the 'Pfeiffer doctrine' on the microbial origin of influenza and the illusion of a world free of infectious diseases. This was an illusion destined to surface again at the end of the century and collapse with the advent of AIDS.

The rapid changes that syphilis underwent after the first major outbreak that occurred in Naples in the mid-1490s are believed to constitute the first well-documented example of a human disease. The new plague was of exceptional virulence, highly contagious and causing severe ulceration at the site of infection. According to medical and other historical sources, the 'genius epidemics' changed some years after this onset, and a slower-progressing form of syphilis seems to have replaced the initial severe form, as do many virulent epidemic infectious diseases that appear in devastating forms in a previously uninfected population. But what exactly were the features of the disease at the moment of its appearance in Europe at the end of the fifteenth century? How many years did it take for the early, virulent form to be replaced and become endemic? What was the pace of these changes through the decades following the onset of the epidemic? In this essay, I review these issues through an analysis of a large number of chronologically-ordered primary historical sources.

Malaria, considered almost eradicated in the mid-1940s, is still a worldwide public health problem.

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