Lewis was in Tanzania to raise money and awareness about the need for clean water in East Africa. He was set to do the climb with former Chicago Bears defensive lineman Tommie Harris and several others.
"Ray stood all day yesterday doing a hearing mission," trip organizer Frank Gamble said in a video, via The Baltimore Sun, summarizing the climb so far. "Last night, he had a bad night, fevered and really rough. So this morning, when he woke up, the foot was killing him, years of injuries and all of that. So we're going to miss him."
Lewis, 38, is a two-time NFL Defensive Player of the Year who retired after winning his second Super Bowl with the Ravens in February. He now works for ESPN as a football analyst.
"The greatest thing we have in life is called opportunity," Lewis said. "It's what you do with opportunity that will actually leave a lasting legacy. This team we put together to go impact lives after lives and lives and to touch people that may never know our names, but will remember our legacy because of what we did to bring clean water to the motherland back to unfortunate people to give them a second opportunity to say, 'You know what somebody thought about us even when we didn't have an opporutnity to think about ourselves."