An honest-to-Merlin laugh was startled out of Snape. The fact that Al was actually capable of cracking a joke, when he'd so recently been so anxious... well, it was impossible to resist reaching out to tenderly brush a bit of that impossible mop of hair away from his glasses: a brief, undemanding moment of tenderness.
But when Al started to talk about making himself someone capable of denying the diagnosis, his smile faded into an intent, searching look.
"I doubt anyone else from the wizarding world will think to consult Muggle references: I understand Brocklehurst was highly unorthodox in that respect. Just ask Molly Weasley about her reaction when she heard of the unsuccessful Muggle 'treatments' tried on Arthur's snakebite.
I was proud of you, by the way, for not having an outburst the other night," he added, seemingly changing the topic while in reality doing nothing of the kind. "I gather that you've got that well under control these days?"