A while ago, I posted a query to @Icipeinsectscie and got a somewhat reassuring response from one of the scientists at ICIPE who do research on tropical diseases.
Yes, Nairobi is becoming a home of many mosquitoes but mostly culex mosquitoes – they do not transmit malaria but are a biting nuisance. The main reason for their increase is due to increasing urbanization which is taking place without considerations of health and safety issues. Urbanization causes changes in land use and creates breeding sites for mosquitoes and other public health nuisance insects.
People need to be aware of this and take measures to control them - like sleeping under a net, and using proper sanitation. For further details, please see the quick tips to control mosquitoes and malaria.