A mosquito net offers protection against mosquitos, flies, and other insects, and thus against the diseases they may carry.
Examples include malaria, dengue fever, yellow fever, and various forms of encephalitis, including the West Nile virus.
To be effective the mesh of a mosquito net must be fine enough to exclude such insects without reducing visibility or air flow to unacceptable levels.
It is possible to increase the effectiveness of a mosquito net greatly by treating it with an appropriate insecticide or mosquito repellant.