"Zombies? Oh- you mean Inferi. Dead bodies reanimated by a Dark wizard to do his bidding," said Sirius. Then he sighed. "Look, I’ll be honest. I have no idea what this means. It could be visions of the future- or it could more than likely be nothing," he hastily added when he saw Harry’s stricken look. "But don’t worry about it too much. If it keeps getting worse then maybe talk to Dumbledore or someone you trust about it. Hermione’s smart, she actually might know something I don’t."