Complex sentence: Miasma was believed by most to be the vector of transmission of cholera, which was on the rise in 19th century Europe, and was deeply feared by all because of the speed it could spread, and its high fatality rates.
Split into simpler sentences: Miasma was believed by most to be the vector of transmission of cholera, which was on the rise in 19th century Europe. The disease was deeply feared by all because of the speed it could spread and its high fatality rates.