In a long statement on the band's website, front man Jon Foreman announced the "sobering news".
He said: "I got a call from Jerome a few weeks ago. He was calling with sobering news: he has cancer.
"One of Jerome's kidneys has a large mass on it that has been growing for quite some time.
"The good news is that the cancer has not spread to his other organs. Removing one of his kidneys will hopefully be the first and final cure for Jerome's fight with cancer.
"The surgeons and the medical team have been incredibly accommodating, making sure that the surgery was scheduled as soon as possible."
Fontamillas, 51, joined the band back in 2003.
In a note to fans, he said: "We've been through so much as a band. This is just one more mountain to climb, one more storm to weather. I know we'll get to the other side.
"And I look forward to having more reasons to be thankful when all of this is behind us. We will give you an update when we have news to tell you.
"Thanks again for years of love and support. Merry Christmas and Happy New Year to all of you, we look forward to telling you some good news in the near future."
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