His aversion to heretics and infidels was so great

The faith of Camillus was likewise show in the aversion which he always had for infidels. So that when he had occasion to speak of the heresies that were then so widely spread in France, Germany, and England, especially against the obedience due to the Holy See and the Roman Church, he would lift up his eyes to heaven and cry out with tears: "Is it possible that men should be so blind and not see the truth of our faith?...."

His aversion to heretics and infidels was so great that he seemed to know them by their smell. Thus, when he was once traveling from Milan with a large company on horseback, he conversed freely with all but one, who he said smelt like a heretic; and so indeed the man turned out to be. He remembered the counsel of St. John, not even to salute or eat with infidels, and so would have nothing to do with them or with Jews, especially with those who showed no respect at all for our religion.

The Life of St. Camillus of Lellis by Father Sanzio Ciccatelli, trans. by Father Frederick Faber. p. 207