If it's influenza, as was reported a few weeks ago when DGL (I think it was him) played through it, then we as a society know a lot about how it spreads and how to help avoid it. It is spread by respiratory droplets in close contact with others. The sort of things a person generates when they run and breath hard. So if you say "hey, man, why don't you sit out a few days and come back when your fever is gone," then you stop that person from infecting his teammates.
And for young, healthy people, flu tends to knock you down for 2-5 days and then they can rejoin polite society.
If you instead say "wow, way to be tough and fight through it," then you're probably infecting a new person every few days and you have a wave of illness run through the team.