Cancer.
Personal life
Colmes was married to Dr. Jocelyn Elise Crowley, a professor of public policy at Rutgers University, whose sister is Monica Crowley, the conservative radio commentator, pundit and television personality.[17]
Colmes died on February 23, 2017 at the age of 66 after suffering a “brief illness,” which Stuart Varney disclosed to be cancer.
Source: Wikipedia
I saw him on TV a few months ago, and he was looking even more thin than usual. The reports said a brief illness; maybe they only found the cancer recently.