‘The conclusion which seems to emerge from this analysis of the constitution of the primary wave is that its onset and progress were not very different from the onset and progress of epidemics which have occurred under similar conditions in other parts of the world where the bulk of the population at risk possessed little or no natural or acquired immunity to the disease. Among several examples which might be quoted, Dr. Wenyon has reminded me that an epidemic in the town of Nemi near Rome, which we visited together a few years ago, was perhaps as instructive as any. Nemi is one of the so-called " castle towns " in the Alban Hills. It is situated on Lake Nemi, at no great distance from endemic foci of malaria in the Agro Romano but ordinarily, except for imported cases, it is quite free from the disease and anopheles are very rare. In 1928 and 1929 the level of Lake Nemi was lowered about 14 metres in a search for the sunken barges of Caligula. This work created numerous suitable breeding places of anopheles which became very abundant. The result was that in 1929 there was a serious epidemic in which 616 persons out of a population of about 1,000 were affected. This epidemic, like the epidemic in Ceylon, showed that in an area where there are few gametocyte carriers and almost no anopheles, an invasion by great numbers of these insects can give rise. to a serious epidemic provided that the population at risk possesses no immunity to the disease. It is equally true that areas in which there are plenty of anopheles but no gametocyte carriers may almost immediately suffer from an epidemic when a number of gametocyte carrying cases are imported. This is what happened in England and various countries in Europe after the War. In both conditions, for reasons which I mentioned in the discussion on the Ceylon epidemic at a previous meeting of this Society, lack of immunity in the population at risk is almost certainly an essential factor.’ (892)