People flying sick has been a problem for a long time. Some of that is because of airline policies – it’s costly to change your plans, and a doctor’s note won’t even help you. Much of it is more broadly cultural. We expect people to ‘tough it out’ and colleagues don’t want us to ‘let them down’.
Before the pandemic the CDC actually recommended foregoing travel if you had a fever over 100 degrees and other symptoms, but a fever alone wasn’t reason to cancel a trip in their medical opinion. And about half of people said they’d fly even if they had the flu. That really needs to change.