I got it too. The information could be a tad more specific, lol. "Europe" is a pretty big place and they are not really telling us what the real problem is. Big surprise there, of course, lol.
Historically, the only place I remember being very prone to mosquitos on previous trips was Florence. Screens never really caught on in Europe though, so if you are staying at a B&B or charming inn, that is good to know.