Calchas knew the reason why the Greek soldiers were dying from a plague during the Trojan War but he held his tongue because Agamemnon was the cause for it. It wasn't until the other chieftains, Odysseus and Achilles included, did he ask for protection and tell everyone that it was Agamemnon and his taking of Chryseis, daughter of one of Apollo's priests, that caused the plague. In fact, it was Apollo himself that gave the plague to the Greeks.