Ronny Jackson, who was the White House physician to former Presidents George W. Bush, Barack Obama and Donald Trump, has joined a growing legion of Americans (and medical experts) who are gravely concerned about the physical and mental health of President Joe Biden. Jackson — a retired Navy rear admiral who now represents Texas as a Republican congressman — sounded the alarm on Sunday when he said Biden’s lack of public appearances is a “MAJOR red flag” signaling that something is seriously wrong with the 78-year-old career politician. Biden has yet to hold a formal news conference as president. “I served as White House physician under THREE Presidents,” Jackson tweeted. “I’ve seen what it takes physically AND mentally to do the job.