From The Bird Flu: A Classical Perspective, Gabriel Weiss, 2007, p. 11
The Book of Rites was written during the Warring States Period and then extensively reworked during the first century BCE. Like the Rites of Zhou, this work describes social decorum and ceremonies of the Zhou Dynasty. The larger context of the line translated above is made explicit, and we discover that it is unseasonable weather or a confusion in the energetic pattern appropriate to a season, in this case winter, which brings about pestilence as well as other disasters:
When in the second month of winter <midwinter> the mandate of summer is expressed <there is unseasonably summer-like weather>, there will be a drought throughout the land. The atmosphere will be misty and dim and the thunder will rumble. Should the mandate of autumn prevail <unseasonable autumnal weather>, there will be rain and wetness, the edible gourds will not mature and the country will be beset by battles. If, instead, the vernal mandate is expressed during this time, then locusts will make waste and the water-sources became salty or dried up. The people will suffer many diseases and pestilences 癘.
The theme of phenological aberrations linked to meteorological abnormalities is reiterated: a relationship is drawn between a perturbation in climate, here a sequence of odd weather in the course of a year, and the advent of periodic biological phenomena. Phenology is the study of the relation between climate and periodic biological phenomena. Rotting crops and ravaging insects can easily be traced back to the effects of climate conditions. The trail of influenza is not so easily traced back to its roots in the patterns of climate.