We have noticed in Chapter 4 how diseases affecting crops, and others attacking animals and the human population more directly, made parts of the fourteenth century and probably most of the fifteenth century an unhealthy period in European history, as appears very clearly from various works on the Black Death (e.g. Twigg 1984, ,Ziegler 1970). This also is obvious from the literature of the time even so long before the keeping of medical statistics, but seems to be substantiated by such studies as those of burial remains in England which indicate a reduction of the average length of human life from 48 to 38 years between AD 1276 and 1400 (Comfort 1966). We have discussed briefly in Chapter 4 the breakdown of the earlier mediaeval warm climate. Further details of the meteorology of the breakdown are examined in Lamb (1987) . Diseases, and the vector organisms which transmit them , each have their distinctive optimum conditions for existence and for flourishing. A majority of the moulds which affect crops grow at temperatures between 15 and 30 degC, but dryness limits their growth. No mould appears to grow in stored grain that has a water content of 13 per cent or less, hence the great importance for the economy and for human health of the modern drying of cereals before storage (Matossian 1984a).