Physicians could, therefore, only advise public purifications of the air by means of large fires, as had often been practised in ancient times; and they were obliged to leave it to individual families either to seek safety in flight, or to shut themselves up in their dwellings, a method which answers in common plagues, but which here afforded no complete security, because such was the fury of the disease when it was at its height, that the atmosphere of whole cities was penetrated by the infection.
Of the astral influence which was considered to have originated the "Great Mortality," physicians and learned men were as completely convinced as of the fact of its reality.

A grand conjunction of the three superior planets, Saturn, Jupiter, and Mars, in the sign of Aquarius, which took place, according to Guy de Chauliac, on the 24th of March, 1345, was generally received as its principal cause.

In fixing the day, this physician, who was deeply versed in astrology, did not agree with others; whereupon there arose various disputations, of weight in that age, but of none in ours.

People, however, agree in this--that conjunctions of the planets infallibly prognosticated great events; great revolutions of kingdoms, new prophets, destructive plagues, and other occurrences which bring distress and horror on mankind.

No medical author of the fourteenth and fifteenth centuries omits an opportunity of representing them as among the general prognostics of great plagues; nor can we, for our part, regard the astrology of the Middle Ages as a mere offspring of superstition.