A lot of pathogens that beset us today did not exist in 742 AD. AIDS is one obvious one, the common cold was not recorded prior to 1660. The plague only goes back to 1340, malaria maybe 1600. In prehistorical times, deadly pathogens would kill off a local population and then, they would die out because there was no one left to infect. Diseases like smallpox and leprosy persisted because they took longer to do kill a person. We are the descendants of the survivors, the people who had resistance to the worst pathogens. Native Americans, for instance, did not, so when they contacted Europeans, their populations had not been through the winnowing process.
The major Plague epidemics occurred in 540 at Pelusium, Egypt, reached Constantinople in 542 and spread into Europe and Asia (the Plague of Justinian) in the following decade; 14th century Europe, following the caravan routes, it was in the lower Volga River basin in 1345, the Caucasus and Crimea by 1346, Constantinople by 1347, Alexandria in the autumn of 1347, Cyprus and Sicily in that year, Italy by winter 1347, Marseilles by January of 1348, Paris in spring 1348, followed by Germany and the Low Countries in that year, Norway in May 1349, eastern Europe by 1350, and finally Russia in 1351, but smaller outbreaks continued for about 200 more years; Austria in 1711; the Balkans from 1770-1772. The last major pandemic ran from 1855-1896 worldwide, but mostly in China and India, wherein more than 12 million died. Manchuria in 1910–1911 witnessed about 60,000 deaths due to pneumonic Plague with a repeat in 1920–1921; and a minor outbreak occurred as recently as the summer of 1994 in Surat, India closely following an earthquake in September 1993.
From 1150-1200 there was a major warming throughout Europe. This, coupled with the rise of the mercantile class, led to improved diet and greater population growth. By 1340, Europe was significantly overpopulated. This was followed by the so-called Little Ice Age, which ended by 1351. The resulting climate was colder and wetter than normal. With population higher than it had been in some time, and crop yields reduced, per capita caloric intake fell precipitously, general health declined, and the pest population increased. Not a welcome combination of circumstances.