I have not been sick much in the last few years - but just this Sunday I came down with a pretty good sinus cold - not flu or anything, just hte head stfuu, bit of caugh, you know
I almost always swim through this stuff. BUT>>>
It is tough - I swim slow, and my HR tends to go up high even with just moderate efforts. I mostly do longer swims, nothing taxing - just keep moving. I also feel like being in the water clears my head out.