I did a trail marathon in October in the Lakes, thinking I just had a bad chest infection. I wasn’t unable to run up any incline whatsoever and was that dehydrated that I was filling my water bottles from streams along the route.
I ended up an hour and a half past my target finishing time lol
I've heard there's loads of gnats piss in those streams during that time of the year mate so I reckon your immune for about 100 years if you filled your water bottle multiple times