Imagine throwing mosquitoes out and expecting them to stay out lol.. They never wish to leave any house they are in expect if they are killed in it. If you leave them, they will such your blood, disturb your sleep and keep singing in your ears every night.
Mosquitoes are the biggest problem that we have in most African countries because they have infested so many places that no matter the kind of insecticide that you are using, within a few days, they are going to swarm up and still start disturbing people and biting them to transfer diseases to them.