How does libel law work?

A person is innocent until proved guilty. What are the implications of this if a person is accused of libel? As an example, Michele Bachmann recently said that she knew of a case where the HPV vaccine caused mental retardation. Suppose that the manufacturer of the vaccine, Merck, brought a case against her. What would Merck have to establish in order to get paid?

Would they be required to provide evidence that the drug does not cause mental retardation? Would they have to show that Bachmann knew that the statement was false, or would it be sufficient to show that she had insufficient reason to believe that it was true? Would Merck have to provide evidence that they were damaged by the charge? If Merck could not prove libel, could they still get an order to have Bachman not repeat the statement until such time as she had sufficient evidence?