Put the textbook's illustration into standard form:
Minor premise: Zayd has corrupted humors.
Major premise: In the historical theory assumed, everyone with corrupted humors has a fever.
Conclusion: Zayd has a fever.
“Corrupted humors” links the premises and, within that theory, is treated as the real cause of fever. It thus serves as both ground of inference and explanation of the occurrence.
Analyzing the conditional logical form is distinct from endorsing the historical medical major premise as a present-day scientific fact.