The mosquito does not originate the malarial infection; it simply serves as the temporary host of the micro-organism (Plasmodium malarioe) which is the cause of the disease, having obtained its transient "guest" from some human being.

Consequently, marshy districts that are full of mosquitoes are not malarious unless the mosquitoes are of the kinds capable of lodging the plasmodium, and unless there is or has recently been present in the neighborhood some person affected with malarial disease.