Quick google brings this up.
Dengue fever - Wikipedia, the free encyclopedia
It can feel like bad flu (as can malaria). In fact my doctor back home once misdiagnosed my son's malaria as flu in spite of me saying it wasn't and reminding him I'd just come back from a year in Africa. Doctors in Europe rarely see any tropical diseases so they often don't know that much about them.
Dengue can be fatal when it turns into dengue haemorrhagic fever but most people who catch dengue survive.
I'm surprised that the islands BobMac mentions are such high risk areas... I thought dengue was primarily an urban disease? There are certainly dengue cases in BKK every year. Also surprised that there has been such a severe outbreak now, as it's usually worst during the rainy season when more mosquitoes are breeding.