Sorry girls. Can't find the off switch.

I just got a call from the doc. Ethan has a urinary tract infection! He has shown no signs of being sick except for a slight temp yesterday but it was a really hot day. Maybe this is why he is not sleeping well. The doc has scared me slightly because she rang the paed (who couldn't see me for Ethan's first appointment until the 25th Jan) and they have pushed him in for tomorrow. I know I shouldn't worry because he is still fairly happy but I stress at the slightest of things. How could he have caught the infection? Anybody got any ideas?