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.
From The Bird Flu: A Classical Perspective, Gabriel Weiss, 2007, p. 10
The context in which the concept of 癘 was used in ancient texts reveals something of the general understanding of the term at the time. For instance, in the early cosmological text the 淮南子 Huainanzi, a philosophical classic from the 2nd century BCE, we read in Chapter 5, “Time’s Lessons 時則訓”, the line
When spring’s mandate is expressed, then insects and larvae cause great loss, the water springs became salty and dried up, and the people experience many acute illnesses and pestilences 疾癘.
 John S. Major, Heaven and Earth in Early Han Thought: Chapters Three, Four, and Five of the Huainanzi (Albany: SUNY Press, 1993).
From The Bird Flu: A Classical Perspective by Gabriel Weiss, 2007, p. 5.
In Section 1 of The Rites of Zhou 周禮 , “Offices of Heaven 天官冢宰” we read
Physicians treat the pathology of the ten-thousand people. All four seasons 四時 have their pestilences and epidemics 癘疾: spring has the disease of wasting-of-the-head 痟首疾, summer has the disease itching-sores 痒疥疾; fall has diseases of malarial fever cold 瘧寒疾; winter has the disease of coughing due to uprising qi 嗽上氣疾. By means of the five flavors, the five grains, and the five medicines, the diseases can be managed, and by means of the five different energies, the five sounds, and the five colors, whether the patient will live or die can be ascertained. Both treatment and diagnosis depend upon the changes of the nine orifices, and observation of the movements of the nine storehouses. As for diseases of the people, one should first categorize analytically, and then treat. Should the case end in death, each method employed should be recorded and submitted to the Official Physician. 
In this passage, the four seasons are immediately correlated with different categories of disease, without seeking to explain any causal relationship. The quality of the illness or epidemic is, however, related to the season in which it appears.