In 1854, John Snow identified the first recorded cholera cases in Soho, London. He identified the water pump on the street as the cause of the outbreak since everyone who got sick was drinking from this pump. This led to a revolution in understanding public water supplies need to be cleaned or risk disease. In this small outbreak 616 people died within one month. London had several severe cholera outbreaks betwee the years 1832 and 1866, which is why it was so feared during this time. The symptoms of cholera include being dehydrated, due to constant diarhea, which led to blue-tinted skin from oxygen loss, muscle spasms, and possibly death. A cure for cholera was not discovered until the late 1880s. 

Polio was another horrible illness affecting many in the Victorian era, but its first known recorded case was in Ancient Egypt. London's first record of polio came from a pediatritian named Michael Underwood who described the paralyzation and deformation it caused in infants in 1789. The next few reported cases jumped around from Norway, Sweden, and Vermont. This illness comes from poor hygeine, and thus will be present in any unclean place which is why it jumps around so much. The terror of getting polio continued until 1955 when a vaccine was finally discovered. 

Both of these illnesses contributed to a very high mortality rate in the Victorian era, especially in children. Throughout the novels we've read, Cholera, Polio, and other illnesses (Tuberculosis/Consumption) have been very present and feared. Both of these diseases spread through lack of hygiene and cleanliness, which is why it is so difficult to identify a source. These illnesses became less and less common over time becuase general sanitation habits increased. 

Sources: 

https://www.branchcollective.org/?ps_articles=pamela-k-gilbert-on-choler...

https://www.bl.uk/collection-items/john-snows-account-of-the-cholera-out...

https://www.passporthealthusa.com/2017/11/history-of-the-cholera-vaccine/

https://www.britannica.com/science/polio/Polio-through-history