US President Joe Biden tested positive for Covid-19 on Thursday, with his doctors saying the 79-year-old leader was experiencing only mild fatigue and would isolate at the White House while carrying out his duties. Biden, who posted a picture of himself working at his desk, tweeted that he was “doing great” – and apologized for missing a scheduled event in Pennsylvania. “Keeping busy!” added the US leader, in a post carefully curated to show he remains at the country’s helm – smiling in a suit, with smartphones and documents laid out in front of him. An update from me: pic.twitter.com/L2oCR0uU…