In the Iliad, book 5, the Olympian god of war Ares is wounded by mortal hero Diomedes. Ares is to Olympus in a hurry, where Paeon applies medicine that gave an instant relief. Hades too was helped by Paeon when he was shot with an arrow by Heracles. Hesiod says Paeon is an individual god and not a group of people.
In time, Paeon (more usually spelled Paean) became a name used for Apollo. This is because Apollo was able to bring disease. Later, Paeon became a name used for Asclepius, the healer-god.
References
↑Liddell and Scott, "Paean"; Connor, p. 1069; PA-JA-WO in Linear B at Mycenaean Knossos attests the name as an individual Mycenaean deity (Chadwick, John. The Mycenaean World. Cambridge University Press, 1976, p. 88).
Homer. The Iliad with an English Translation by A.T. Murray, Ph.D. in two volumes. Cambridge, MA., Harvard University Press; London, William Heinemann, Ltd. 1924.
Homer. The Odyssey with an English Translation by A.T. Murray, PH.D. in two volumes. Cambridge, MA., Harvard University Press; London, William Heinemann, Ltd. 1919.
Liddell, Henry George; Scott, Robert (1940). A Greek-English Lexicon. Oxford: Clarendon Press.