David Koch is retiring from Koch Industries “and other organizations” due to “declining health,” according to a letter written by Charles Koch and sent to Koch employees Monday. David Koch was hospitalized in 2016 and his health has since “continued to deteriorate,” rendering him “unable to be involved in business and other organizational activities,” the letter says. “His guidance and loyalty, especially in our most troubled times, has been unwavering,” Charles Koch wrote. While the letter does not include specifics about his heath issues, it said that “David has always been a fighter and is dealing with this challenge in the same way.” CNBC reports that David Koch “was diagnosed with prostate cancer more than two decades ago,” and has since become a board member of the Prostate Cancer Foundation. This comes a day after Koch Industries’ political network announced a plan to battle the Trump Administration’s tariffs against allies and other countries.