1 : a[n illness] (such as malaria) marked by paroxysms of chills, fever, and sweating that recur at regular intervals
2 : a fit of shivering
1 : The symptoms of some ague continued throughout the night and into the following day.
Odd that this is listed as a two meaning word. Also, the official definition says “a fever” and then describes that it involves fever and chills. I swapped illness in as an attempt to remove confusion. Fever can mean high temperature, and refer to illnesses. It’s another word that’s not in current dection, so I suspect it was added like many to provide GMs a proper reference for reading materials, or for giving a NPC a distinct diction.