So this is actually shockingly similar to something that happened to me. I did a lot of back country travel with sketchy water and developed something close to dairy intolerant for more than 10 years to the point I wasn't sure if I had always had it. Eventually I got lucky on a test (for E. Coli not giardia) and the treatment was to go to nuclear on my intestines. That week wasn't fun. Now i can eat ice cream and dairy with out trouble. I'm not convinced that test was related to the 10 years of symptoms, but the hard reset on the gut definitely worked.
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