Answer:
The cause of dengue is being bitten by a mosquito which has taken a blood meal from an infected carrier, usually human. Much the same as Malaria, which is only transmitted from human to human by the mosquito. West Nile Virus, EEE and other "flu like symptom" illnesses can be transmitter by mosquito from a human, bird, horse etc.
The problem with Dengue is if you get it the second time it can be deadly.
Check out CDC: See Related Links
Click on Florida to see how bad it really is.
What we need is a non-toxic pesticide that also keeps repelling mosquitoes for more than a few hours.