On November 3, 2015, Bad Brains announced on their Facebook page that Dr. Know (Gary Miller) was hospitalized and on life support, after many other musicians reported so. Bad Brains later announced, on November 10, that Dr. Know had come off life support and was "under close care" after a heart attack and subsequent organ failure. His bandmates were asking fans to help via a GoFundMe campaign to pay his expenses for rehabilitation. After nearly three months in the hospital, he was transferred to a medical rehabilitation facility for the physical therapy and other necessary treatment he needed to make a full recovery.  On March 15, 2016, it was reported that Bad Brains frontman H.R. was diagnosed with a rare type of headache called SUNCT, and was seeking $15,000 to fight the "Suicide Syndrome" using methods not covered by health insurance; as a result, a GoFundMe page was created. According to the GoFundMe page, H.R. had dealt with "several health issues" in recent years that he had been able to overcome.  In a December 2016 interview with Rolling Stone, where Dr. Know and bassist Darryl Jenifer talked about the band members' health issues and the status and future of Bad Brains, it was revealed that the band hopes they will record the follow-up to Into the Future, titled Mind Power.  On June 8, the band played an unannounced short gig in Darryl Jenifer's art exhibition. They played three songs with H.R. on vocals, two songs with Lamb of God frontman Randy Blythe on vocals and one song with Sid McCray singing with the band for the first time in 39 years.  On April 2017, it was announced the Bad Brains would play an exclusive 40th anniversary set at Riot Fest in Chicago's Douglas Park. On September 16, 2017, they made that Riot Fest appearance, playing ten songs with H.R. on vocals and three songs with Randy Blythe on vocals.

Answer this question "What was his illness?" by extracting the answer from the text above.
a heart attack and subsequent organ failure.