My favorite insect research subject is the mosquito, but now I'm about to pick up my future vehicle, and I don't want to deal with it in matter of likeness, so I'm to take all related factors into consideration.
Hi Ayman! I also work with mosquitoes. The diseases they may transmit depend on the species and the locality. For example, Aedes for yellow fever, dengue, chikungunya; Culex for Japanese Encephalitis, filariasis, St. Louis encephalitis, avian malaria; Anopheles for malaria, lymphatic filariasis; also Mansonia for lymphatic filariasis. Some diseases are restricted to the tropics while some can be widespread.
If you are going to use laboratory-reared mosquitoes, they are likely to be disease free. However, you must take all precautions when working with wild ones, or during field collections. Peak biting hours vary for different species -- Aedes are diurnal while Culex are mainly nocturnal, although they may bite the entire day.
Mosquitoes can transmit more than fifty diseases. Majority of them are viral diseases. See my article: http://www.ias.ac.in/resonance/Volumes/06/05/0048-0055.pdf
I believe the question is very hard to answer. In addition to all the human vector borne pathogens you have the pathogens that are spread between animals and between animals and humans. Most of the emerging diseases are vector borne and many of them are mosquito borne. There are probably several vector borne pathogens that are yet undetected. If you are looking into a future career I can assure you that medical and veterinary entomology is a fulfilling way to go with never ending challenges and rewards!
As Anders Lindström rigthly pointed out, what we know about vector borne[mosquito] diseases is a drop in the ocean. There may be much more without detection. Medical and vet entomology is a real challenge. lalitha kabilan.