Dengue Fever
How come Dengue Fever is passed on to humans by the Aedes aegypti mosquito and yet not infect the mosquito?
There are many diseases which are transmitted by mosquitos but do not infect them, such as malaria. These viruses (Virii?) are target-specific in that they infect only certain organisms, like humans, and not ants or bees or dogs, etc.
Sort of off topic, but not really: there is currently an "Almost" epidemic in India of dengue fever. 20 people dead, and over a hundred cases so far.
Vaccine For Dengue Fever Shows A Glimmer Of Hope
In a study with about 4,000 Thai schoolchildren, a vaccine for dengue fever works well against some strains of the dengue virus. But the overall level of protection was lower than hoped for. The results suggest that a vaccine for dengue fever can be developed eventually.
Source: Science
Mosquito saliva increases disease severity following dengue virus infection
Insects transmit diseases when, probing for blood vessels, they inject saliva together with viral, bacterial, or parasitic pathogens into the skin of mammalian hosts. A study in mice suggests a critical role of mosquito saliva in the outcome of dengue virus infection. Ref. Source 7d.
A protein in mosquito spit can keep Dengue virus in check
Mosquito saliva influences transmission of viruses to a bitten mammalian host. For example, it contains factors that dampen the host immune response and so facilitate infection. A study reports on a saliva protein with the opposite effect: D7, a protein present in Aedes aegypti saliva, binds to Dengue virus (DENV) and inhibits its transmission to human cells and mice. Antibodies against D7, which are present in humans exposed to mosquito bites, might therefore facilitate virus transmission and enhance disease severity. Ref. Source 7k.