<PAGE>


                                                                    Exhibit 99.1

                                                              Page 9 of 10 Pages

                                  SCHEDULE 13D

CUSIP No. 59156R108


                             JOINT FILING AGREEMENT

   Pursuant to Rule 13d-1(k)(1)(iii) promulgated under the Securities Exchange
   Act of 1934, as amended, each of the undersigned agrees that the Statement on
   Amendment No. 50 to Schedule 13D, to which this exhibit is attached, is filed
   on his or her behalf.

   Dated: August 7, 2012

                *
   ----------------------------
   Sylvia Mathews Burwell

                *
   ----------------------------
   Cheryl W. Grise

                *
   ----------------------------
   R. Glenn Hubbard

                *
   ----------------------------
   Steven A. Kandarian

                *
   ----------------------------
   John M. Keane

                *
   ----------------------------
   Alfred F. Kelly, Jr.

                *
   ----------------------------
   James M. Kilts

                *
   ----------------------------
   Catherine R. Kinney


<PAGE>


                                                             Page 10 of 10 Pages

                                  SCHEDULE 13D

CUSIP No. 59156R108


                *
   ----------------------------
   Hugh B. Price

                *
   ----------------------------
   David Satcher, M.D.

                *
   ----------------------------
   Kenton J. Sicchitano

                *
   ----------------------------
   Lulu C. Wang


            * By /s/ Nicholas D. Latrenta
                 -----------------------------
                 Nicholas D. Latrenta
                 Attorney-in-fact