How would people in the 1840s explain diseases?

From ancient times until well into the 19th century, people thought the human body was composed of four "humours" and disease was caused by an imbalance of them.
ocp.hul.harvard.edu/contagion/humoraltheory.html

The discovery of hand washing led to a germ theory of disease, but it didn't catch on until all the believers of the humour theory had died.
en.wikipedia.org/wiki/Germ_theory_of_disease