Pony,
I fail to see the relevance of West Nile Virus and Malaria other than that both are spread by mosquito bites. The US has several contagious diseases, just as every nation does. Compare the Rabies infection statistics in the US with those in the Europe to see a scary reality - we are far less 'civilized' when it comes to prevention of some diseases than the rest of the world (~7400 human infections per year compared to ~5).
Debating the merits of taking preventative medications on a Scuba forum is nearly pointless. The CDC recommends taking the meds. DAN recommends taking the meds. Your doctor may or may not recommend taking the meds. If someone choses not to that is their choice, but it is against prevalent medical advise.
The odds of getting malaria in the Bay Islands are lower than many parts of the world, but are measurably higher than in the US or Europe. Most of us go to the Bay Islands on holiday and wish to come home in better or equivalent condition to how we left... taking the chance of contracting malaria seems foolhardy to my simple mind.
Those who decide not to take the meds do so at their own peril. Will, who has posted earlier in this thread, does not take the meds and has contracted malaria. While his case was not as severe as others, it should stand as testament to the potential for anyone to catch it. I trust and respect Will a great deal, but I choose to take the meds and will continue taking them when we go to Roatan to complete DM and IDC.
To each his/her own, but if you don't take actively try to prevent malaria, don't be upset if you get it. Lastly, if you use DEET to protect against bites please have the decency to wash it off before diving - the reefs take enough abuse from the bloody cruise ship passengers.
