Who poisons Ron Weasely? How?

Weasley drinks some of Romilda's love potion, so Harry takes him to Slughorn hoping to get him an antidote. Slughorn posions Ron with the poisoned-laced mead, and Harry saves Ron's life.

But Slughorn didn't know that the mead was poisoned so how could he poison him. I think no one really poisoned him.

Actually, it wasn't Slughorn that poisoned Ron. The bottle of mead that Slughorn had was supposed to be for Dumbledore, who Draco Malfoy was given the task of killing. The bottle somehow reached Slughorn, and he, not knowing that it was poisoned, gave it to Ron and Harry. So, indirectly, Draco Malfoy had unknowingly almost killed Ron Weasley. Fortunately, Harry Potter used his quick wit and shoved a bezoar down Ron's throat, which they had learned in Potions nullified the effect of poisons.