I was playing against Plague Rat a few days ago with some friends, and a unique situation came up. I believe it should have gone one way, one of the players thought is should go another... So I need to get an official ruling on this.
We were fighting Plague Rat in Magmaria, order was Legacy, Visionary, Adamant Sentinels and Scholar. All players were infected, Medico being the choice for infection for a few extra heals with Hypocratic Oath. I, playing Visionary, put 2 TtE on Medico as well as Legacy double-boosting damage. Plague Locust was not in effect. Legacy played Heroic Interception and thus every other hero target was immune to damage.
Now the big question: If Medico has Hypocratic Oath out but is immune to damage, including self-damage from Infection, should he be able to cast a heal off of that start-of-turn hit?