I hiked half the GR20 last summer.
Mike saw my trip report and comments:
… treat the water. Very often the Corsicans pump their water directly from nearby streams so the water “might” be infected due to the heavy grazing. I’m speaking from personal experience 😦 …
… bed bugs are a recurring problem with most of the refuges … I agree with you. Tents are much safer!
Mike’s got a 55min video documentary on the northern section linked from corsica.forhikers.com/gr20 and the equally detailed southern section sequel on vimeo.
Or you can see both on Mike’s GR20 blog.
Watching those would be ideal preparation for the “toughest long distance trail in Europe”.
besthike.com 
Although I’ve not this GR20 myself, several friends have done it in recent years and have really enjoyed the trail and highly recommended it to me. Looks amazing and one day I’ll have to find room in my busy summer shedule to have a go at it myself.