There has been Dengue Fever on St Barth for longer than I've been visiting the island. I remember seeing warning posters about it in Le Select decades ago. Dengue Fever is likely endemic, and isn't going to disappear until the mosquitoes that carry it disappear, which isn't going to happen. Chikungunya is new to the region, or at least newly recognized, and the press coverage has raised awareness of both Chikungunya and Dengue. It's my impression that the increase in Dengue cases this year is not really any different than in any other year. Many residents have previously had one or more Dengue serotype, which makes them resistant to that same serotype, but Chikungunya is new. The presence of a new virus suggests that there will be more people impacted, at least in the short term.
The island appears to be acting aggressively to respond to the problem. Tons of old tires, which make great mosquito breeding areas, have been removed from the island. Insecticide spraying has been repeated multiple times, as is noted above, much more frequently than in the recent years. Education continues, reminding people to remove potential breeding areas, which are basically anything that holds standing water, even something as small as a bottlecap.
At the end of the day, it's up to each of us to protect ourselves. Awareness, proper use of repellants, and zapping mosquitoes with a skeeter racket are all part of that.