I was talking to some girls last night about making sure they washed their hands after they'd been in contact with water from the brook, and it seemed like a good time to post another reminder here about Weil's disease (leptospirosis). This is a bacteria-carried illness that can kill you if it's not treated in time, and you can catch it through exposure to rat urine. It can enter the body through cuts or the lining of the nose or throat - so if you got your hands wet and then bit your nails, you could swallow some - and the symptoms start off as flu-like.
So always wash your hands if you've been mucking about in ditches or streams; don't eat or drink while you're there; if you have cuts or grazes, make sure they're covered with a waterproof dressing and don't let them touch the water.
More information here: http://www.iphe.org.uk/health/weils_disease.html