Zhao Quan is not ill. He's just a bloody head, taking other people's blood to sell, and he doesn't sell blood himself. This is explained in "Sisters". There are also clues in Favorite: First, Zhao Deyi's father knelt down in front of the villagers and apologized, saying that he would forgive Zhao Quanquan for his son Zhao Deyi's fever. Second, Zhao Deyi told Shang Qinqin that he wanted to sell blood to his brother, who refused. He secretly went to the neighboring village to sell antipyretics. Therefore, knowing that Zhao Quan knows that selling blood will make him sick, he infers that he won't sell blood at all, and naturally he won't get sick.