A friend of mine went swimming in a local lake.  About 12 hours later,
she gets a headache which worsens when she bends over, feels
exhausted, mildly nauseated, etc.  I suggested it might be sinus
problems brought on by contact with polluted water in the lake.  

She said the lake had evidence of vegetation die-off, but it's not in
an area with known run-off from houses, etc., and she doubts that one
could get a reaction this quickly from swimming in the water.

I replied that you can't judge the pollution visually (there could be
a dead deer in the water a few hundred yards away, for example), and
twelve hours seems ample time for an infection (or allergic reaction)
to develop and clog up the sinuses.

Anyone have any thoughts about this or comparable experiences to
relate?