from what I understand about dengue after DH got it - the mosquito (Which is a great stripey thing) tends to be about 250m or less from habitation. Obviously there is no magic barrier. Dengue is much more prevalent in heavily populated areas and there are regular outbreaks. Campaigns go on to reduce the numbers of the mosquito by eliminating breeding areas, which are bits of fresh standing water.
the mosquito also doesn't play fair by a) biting in the day and b) being silent in flight. The bite bloody hurts and you then have 4 days wondering if you are going to get dengue. Any tropical area is a dengue zone, it needs cold weather to kill off the mosquito. BTW that includes Queensland and Hawaii.
we will probably not be returning to tropical areas. You have to be unlucky to get it as a tourist, but getting it twice is too much of a risk for him, and I agree. There are four strains, he is now immune to one of them but the mosquitos are not labelled.
oh, and there's another one:
en.wikipedia.org/wiki/Chikungunya
These diseases are a terrible scourge and a vaccine cannot come soon enough, not for us tourists but for those who have to live with it.
to repeat: 30% DEET (don't ponce around with citronella, it works for about 10 seconds), cover up, use mosquito killers in rooms.