Asclepius's mother, the faithless Koronis, had come from the area of northern Greece known as Thessaly, but Apollo had no reason to be angry with the Thessalians after Koronis had been killed. The one he was angry with in this case was Zeus, so he chose a form of revenge that would affect Zeus much more directly.

On the other hand, Apollo was known to send plagues when it suited him, which is why he was sometimes called by the epithet "Far-Darter." Just such a plague is visited upon the Greek soldiers at Troy in Book 1 of Homer's Iliad: Apollo is angered when the head of the Greek forces, King Agamemnon of Mycenae, steals the daughter of one of Apollo's priests--an ally of the Trojans--and refuses to allow her father to ransom her back.

Supposedly, sudden deaths of all kinds could be caused by Apollo's arrows, or darts, whether from disease or from no apparent cause. When you think about it, it must have been difficult for ancient physicians to explain such circumstances as an apparently healthy man unexpectedly dropping dead from a massive stroke. An angry bow-toting god would provide a helpful etiology.