Mosquitoes transmit important infectious diseases such as yellow fever or dengue fever. Understanding emergence processes and knowledge about diversity of mosquito-associated viruses can help to identify control strategies for these viruses. As part of this thesis, novel mosquito viruses were characterized, and the impact of ecological changes and mixed infections was assessed. A large diversity of 49 viruses from the families Flavi-, Rhabdo-, Reo-, Toga-, Mesoni- and Iflaviridae and the order Bunyavirales was detected in mosquitoes that were sampled in Côte d’Ivoire along an anthropogenic disturbance gradient. The majority of these viruses occurred at a low level, but nine viruses were detected frequently. Five of those viruses showed a clear increase in prevalence towards the disturbed habitats. It was shown that virus prevalence was determined by abundance rates of the main mosquito host species and not by changes in host infection rates. In the mosquitoes from Côte d’Ivoire, a previously unknown alphavirus (Taï Forest alphavirus, TALV) was discovered. The complete genome of TALV was sequenced. TALV represents a suggested novel alphavirus species which belongs to the same phylogenetic clade as the only previously detected insect-specific alphavirus Eilat virus (EILV). Since alphaviruses are rarely found in mosquitoes, additional mosquitoes from Panama were tested. Another novel alphavirus (Agua Salud alphavirus, ASALV) was detected in these samples. ASALV was isolated and characterized in cell culture. As ASALV was temperature-sensitive and could not infect cell lines derived from poikilothermic vertebrates, ASALV is likely an insect-specific alphavirus. Phylogenetic analyses placed ASALV on a solitary branch in basal relationship to EILV and TALV as well as to the arboviruses of the Western equine encephalitis complex. Mosquitoes can be simultaneously infected with different viruses possibly resulting in virus interactions. Mixed infections in cell culture with two co-occurring bunyaviruses as well as ASALV and a negevirus detected in the same mosquito showed no effect on replication. However, the cytopathogenicity of ASALV and the negevirus was increased in mixed infections from which the viruses might profit in vivo. In contrast, herbeviruses inhibited a superinfection with another herbevirus. This ability to restrict infections with a related virus might also influence vector competence for arboviruses. When arboviruses infect humans, they can interact with antibodies that were produced against previous virus infections. These antibodies can cross-react with closely related viruses and enhance the infection. This mechanism was suspected after the Zika virus (ZIKV) outbreak in South America as the seroprevalence of the related dengue virus (DENV) was high and a new ZIKV symptom in neonates (microcephaly) was observed. Placenta explants were infected with ZIKV that was pre-incubated with DENV-immune sera or control sera. It was shown that pre-incubation with DENV-immune sera resulted in a faster ZIKV replication and a higher placenta explant infection rate. An enhanced infection of the placenta can lead to a higher probability of maternal-foetal transmission and contribute to an increase of microcephaly.