Are you sick/ill? 这里sick和ill形容词做表语,可以通用。但是做定语时,只可以用sick如,the sick boy
sick可以做表语,也可以做定语;ill只能做表语,不能做定语,比如只能说You are ill,而不能说You are an ill man.
ill 形容词,sick名词,老外一般都用ill,illness.名词
ill与sick区别是sick多做定语,而ill多做表语
both ok