Arizona Sen. John McCain underwent surgery during the weekend to \u201ctreat an intestinal infection related to diverticulitis,\u201d his office said on Monday.\r\nMcCain, 81,\u00a0who is also battling brain cancer, is said to be in \u201cstable condition\u201d after he was admitted to the Mayo Clinic in Phoenix.\r\n\u201cOver the last few months, Senator McCain has been participating in physical therapy at his home in Cornville, Arizona, as he recovers from the side effects of cancer treatment,\u201d McCain\u2019s office said in a statement. \u201cHe has remained engaged on his work as Chairman of the Senate Armed Services Committee, and has enjoyed frequent visits from his family, friends, staff and Senate colleagues.\r\n\u00a0\r\n\u201cSenator McCain and his family are grateful to the senator\u2019s excellent care team, and appreciate the support and prayers they continue to receive from people all over the country,\u201d his office said.\r\nMeghan McCain, the senator's daughter and current co-host of "The View," noted her father's "intense grit and determination" in a Monday evening tweet.\u00a0\r\n"Thank you to the doctors at Mayo Clinic in Phoenix and to everyone who is praying for him," she wrote.\r\nMcCain, the long-time Republican Arizona senator and 2008 GOP presidential nominee, was diagnosed in July with brain cancer.\r\n\u201cThe prognosis is very, very serious,\u201d McCain said at the time. \u201cSome say three percent, some say 14 percent. It\u2019s a very poor prognosis. So, I just said, \u2018I understand, now we\u2019re going to do what we can, get the best doctors we can find, and do the best we can.\u2019 And, at the same time, celebrate with gratitude a life well-lived.\u201d\r\nAccording to the American Brain Tumor Association, more than 12,000 people a year are diagnosed with\u00a0glioblastoma, a particularly aggressive type of brain tumor. The American Cancer Society puts the five-year survival rate for patients over 55 at about 4 percent.