http://en.wikipedia.org/wiki/Mosquito-borne_disease

I doubt the list in the article is complete, and I doubt whether the full list of infections carried by mosquito is even known to science.

In general, I do know parasites (and diseases qualify) are highly evolved for specific hosts and vectors, so it is not likely a new disease could hitch a ride on a mosquito. It usually takes a long time for the parasite to evolve defenses against both the host and vector's immune defense systems.

I just saw something very interesting on CNN. There is a fungus native to Nepal that invades certain insect nervous systems, altering its behavior so the insect burrows into the ground and dies, and the fungus then feasts on its victim's bodies until it is ready to sprout a fruiting body for its own reproduction. This bizarre parasitic strategy has been described in the Amazon rain forest, where fungi hijack ants, but this is the first time I've heard of it elsewhere.

The Nepalese bug fungus eventually sprouts, and sends a small spore-bearing stalk out, which has long been used in Nepalese folk medicine.
The word has gone out that this fungus has supposed Viagra-like properties, and there is now a problem with foreigners swarming over the alpine meadows trying to dig it up., leaving holes, trampled pasture, and garbage. This is causing conflict between the stockmen who run their sheep on these meadows, and the villagers who have started a bustling cottage industry providing supplies and lodging for the hordes of fungus hunters that have descended on the region.

I guess in nature, we are all hosts and parasites, one way or the other.