Joseph, in his journal, teaches that "Miasma" (malaria) is caused by the climate.
Joseph Smith, Journal, April 13, 1843, The Joseph Smith Papers website, accessed January 3, 2024
The river Missisippi is healthy. unless they drink, it. & it is more healthy then the spring water— dig wells from 15 to 30 feet. and it will be healthy.
There are many sloughs on th[e] Islands. f[r]om <whence> Miasma arises in the summer, and is blown over the upper part of the city.— but it does not extend over the lower part of the city
According to note 271 on The Joseph Smith Papers website:
“Miasma” in the early nineteenth century referred to poorly defined “infecting substances floating in the air; the effluvia of any putrefying bodies, rising and floating in the atmosphere.” Miasma was generally considered noxious. (“Miasma,” in American Dictionary, 529.)