Over the past years, twice after travelling from LHR - LAX - ITO (last leg was always UA1004) I ended up getting some sort of 24h stomach bug starting within a day of arrival. Unable to keep food down, slight fever, ...not too pleasant, fortunately it always subsided after a day or so.
At the time I put this down to lack of sleep/germs on the plane/whatever. But it just happened to my sister too (same flight, same symptoms). I have flown a fair amount of long distance flights and never had these symptoms. So I'm wondering if there could be a particular aircraft (UA use 737s for LAX-ITO) that has some kind of contamination in the water or air or something? Does that sound crazy? Would be curious to see what you other frequent flyers think....