Mosquitoes can transmit numerous diseases, many of which are caused by viruses. This thesis studies two viruses, West Nile virus and chikungunya virus, which have recently caused large outbreaks of neuroinvasive and arthritogenic disease, respectively, in Africa, Asia, Europe and the Americas. West Nile virus is only endemic in Southern Europe even though its mosquito vector, the northern house mosquito, is also common in other parts of Europe. This thesis shows that Dutch mosquitoes are in principle capable of transmitting West Nile virus, but that the ability of the mosquito to transmit the virus is limited due to the cooler northern European climate. Currently there are no vaccines or antiviral treatments available for either West Nile virus or chikungunya virus. We show how chikungunya virus modulates three important immune responses of the human host to complete its replication cycle, which explains among others why generic antiviral treatments such as interferon are ineffective.