The discoveries in the late nineteenth century that malaria is caused by protozoan parasites, which are transmitted by mosquitoes, quickly led to intense speculation about its history in antiquity. The historiography of malaria has passed through three distinct phases during the last hundred years or so. The first generation of historians to consider the effects of malaria did exaggerate its significance in some respects. The argument by W H S Jones that the Greek doctrine of fevers was based on malaria was generally and rightly accepted. However, it is not surprising that his view that malaria was a major reason for the degeneration of the moral character of the ancient Greeks attracted little sympathy. The eradication of malaria from southern Europe in the 1930s and 1940s contributed to a decline of interest in the subject. Subsequently medical historians and even professional malariologists tended to minimize the historical significance of malaria. The revisionist tendencies of this second phase of research led to attempts to reassess some of the details of the evidence upon which Jones had relied. For example, Leonard Bruce-Chwatt and Julian de Zulueta rejected Jones's belief that Plasmodium falciparum, the most dangerous of the four species of human malaria, was already active in Greece in the fifth century BC. They suggested that it started to spread in southern Europe only during the time of the Roman Empire and attributed all the references to intermittent tertian fevers in Hippocratic texts dating to the fifth and fourth centuries BC to the less virulent P. vivax. Although the literature produced during this second phase of scholarship was in many ways more sophisticated, it still suffered from some of the same weaknesses; in particular, analysis proceeded in a purely qualitative manner, without any consideration of the effects of malaria on historical human populations in quantitative terms. A second weakness was a tendency to make generalizations covering the whole of Mediterranean Europe. Since many types of mosquito are incapable of transmitting malaria to humans, mosquito breeding sites do not occur everywhere, and many mosquitoes do not fly further than a few hundred yards from their breeding sites, malaria can only be really understood by micro-analyses, conducted at a very local level, of geography, hydrology, climate, competition between different species of mosquito for breeding sites, and human activities.