창세기 7:2 할라카: Shulchan Arukh, Orach Chayim

5. And they fast for the sick. How so? If a particular illness descends upon many people in the same city, such as diphtheria (meaning a throat-closing disease that closes the depths of the throat, they fast and seal the wells). Or [an illness] like smallpox, and the like, and if people are dying from that illness, it is considered a communal distress, and they decree a fast for it and sound the alarm. Similarly, a choking fever, when a contagious disease spreads, and if it spreads widely among the community, they fast and sound the alarm. However, a dry cough, they only cry out about it. RAMA: And regarding these illnesses, there is no need for them to die consecutively within three days, as is the case with a matter that arises due to a change in the air. Instead, they fast and sound the alarm immediately for these illnesses. (Beis Yosef).