This is a great question! Mosquitoes are not affected by the diseases they carry. When we humans get infected, we can have a fever and feel tired etc, but the mosquitoes show no symptoms. There is no easy way to tell an infected mosquito from an uninfected mosquito as they act and feed the same way. In the lab we have also shown that human cells die when they are infected with some viruses spread by mosquitoes but the mosquito cells do not. It is in the best interests of the germ to keep the mosquito alive so that the disease can be spread to a human. Otherwise they would not be able to complete their life cycle. Perhaps one day we can find a disease that kills the mosquito before it is able to spread the disease to us.