Malarial means relating to or caused by malaria, a serious disease spread by mosquitoes. When doctors describe a malarial fever, they mean the distinctive pattern of chills and high temperature that comes with this illness. A malarial region is an area where mosquitoes carrying the disease are common, typically in tropical and subtropical parts of the world.

Malaria has shaped human history in profound ways. For centuries, malarial swamps made certain areas nearly uninhabitable. The disease killed explorers, slowed colonization of tropical regions, and influenced where cities could grow. Even today, malaria affects hundreds of millions of people each year, mostly in Africa and Southeast Asia.

People once thought the disease came from poisonous vapors rising from swamps. They were partly right about the swamps, just wrong about the cause. It wasn't bad air but mosquitoes breeding in stagnant water that spread the disease. Understanding this connection helped scientists develop ways to fight malaria, like draining swamps, using mosquito nets, and creating medicines to treat infected people.