In the days since the passing of Chadwick Boseman, many have been most surprised about how he was able to keep his illness a secret. No one at Marvel had any knowledge of his illness. Studio head, Kevin Feige was sent an email moments before Chadwick passed but didn't see it until he passed away.
Chadwick kept his illness a secret because he was sure he was going to beat cancer. He was planning to prepare for Black Panther II this month. It was not until a week or so before his death that Chadwick understood the severity of his illness. Only a handful of people outside of family knew of his illness-his producing partner, his agent and his trainer.
Chadwick was also able to keep his illness from movie studios because he found a way to skip the physical exams by doctors before filming.