Why was malaria once associated with swamps and stagnant water?

Malaria was historically associated with swamps and stagnant water because of the miasma theory, a long-held belief that diseases were caused by "bad air" (mal aria in Italian) arising from decaying organic matter and foul-smelling vapors in such environments.1234

Before the role of mosquitoes as disease vectors was discovered in the late 19th century, people observed that malaria frequently occurred in areas with marshes and poor sanitation. Early theories even suggested the disease was contracted by drinking swamp water. Because these environments were breeding grounds for the Anopheles mosquito, the association between the location and the illness was accurate, even though the underlying cause—the transmission of parasites through mosquito bites—was misunderstood.241

This belief led to early public health efforts to drain swamps and marshes to eliminate the perceived source of the "noxious air," which, while based on an incorrect understanding of disease transmission, often helped reduce mosquito populations and limit the spread of the disease.24