I had a negative lab for it, yet the doctor was pretty certain I had it based on symptoms and discussion. Food poisoning doesn't last that long, I had no vomiting, and no other flu-like symptoms. I too was told that false negatives are fairly common. This was based on one stool sample.
I had symptoms for about 2 1/2 weeks before going to the doctor. Fluctuating between the runs and dark, almost greasy stools (not dark like bloody stools though).
I was having waves of nausea. 75% of the day I was OK, but I'd get pretty intense random but short spells of it a few times a day, accompanied by fatigue. The fatigue would often subside and reoccur, just like the nausea.
It was summer and I was trail running about 5 days/week, swimming in three different pools/waterfalls every time I was out, many with high traffic. I also ran out of water on a long run and had to drink about 1 1/2 liters untreated.
I opted for no medication; given I was hydrated and otherwise feeling well, we decided I'd go another week and see what happened, contacting her if symptoms worsened.
They began subsiding in week 3 and I never went back for another test. I haven't experienced anything out of the ordinary since then, although I've heard of the possibility of future flare-ups when untreated.
I've always wondered, is it possible to build an immunity to it? I've always speculated on whether or not frequently ingesting small amounts over long periods of time could build a tolerance...
Summer is here again and I'll be swimming in those same pools/streams many days a week again...It's impossible not to drink more.