Elio comes from a family that partially makes their fortune on medicine. So, he doesn't get sick - to the point it's a rarity. Even if he does, he has an incredibly strong immune system due to constant training, him being an well-trained Aura user, making healthy life choices constantly, and having intermediate knowledge of remedies and medications which he picked up from his elder sibling and read online and in various papers and articles on medical breakthroughs.