Flies and Disease

Written by Morgan Garrett and Lauren Means

“Exterminate The Fly!” Public health flyer from Office for Emergency Management. Office of War Information. Domestic Operations Branch. Bureau of Special Services. (03/09/1943 – 09/15/1945). Wikimedia Commons.

Flies have long been associated with diseases and infections. Flies are easily able to acquire a variety of microorganisms that may cause infections from simply landing on food in an area that may carry thousands of microorganisms, such as a trash can. After picking up an infectious microorganism, a fly can then travel to another food source and may distribute the microorganism by again excreting saliva to dissolve the food or by excreting waste. The fly can be simply carrying the disease-causing organism on its surface or the fly may have ingested the organism with food. The fly can therefore transmit the disease-causing microorganism to an individual by landing on the food or by even contacting the lips or eyes of an individual (Keiding 1986, 306).

During the late 19th and early 20th centuries, there were many questions about the relationship between flies and various diseases. It was observed that flies tended to be prevalent in places affected by outbreaks of certain diseases, but did that means flies caused these diseases?  The main infection associated with the common housefly was diarrhea, a common and often fatal disease of infants. Flies were also linked to typhoid and cholera. Initially, the main piece of evidence that flies might cause all three of these diseases was that these diseases increased during the summer months, when “houseflies are present in great numbers” (Acton 1911). Studies showed the number of flies increasing as the number of cases of diarrhea increased, and promptly decreasing with the decline in reported cases of diarrhea (London 1907). Flies primarily spread enteric diseases, which can be transmitted through contaminated food, water, indirect or direct contact with animals or other infected people. Typhoid fever, cholera, dysentery, and diarrhea are some of the most common diseases that flies helped to spread (Maryleborne 1913). These diseases wreaked havoc in London during the 19th and 20th centuries because the lack of sanitary conditions allowed flies the perfect conditions in which to breed and thus spread disease.


Flies bred in germ infested areas such as sewage, human and animal manure among many other pathogen heavy locations (Hackney 1907).  Due to the tendency of flies to breed in these germ infested areas, they became the perfect carriers of disease. Flies collected germs that caused disease through their frequent exposure to manure, sewage, and garbage, and then subsequently passed those pathogens to humans by excretion in the homes of numerous families. It was common practice during this time to leave food exposed in houses, on kitchen tables or counters, ultimately giving flies the easiest route to transmit their harmful pathogens into the bodies of another species (Hackney 1912). In a case study performed in 1914 there was “a high degree of infection found among flies obtained from the neighborhood of decaying animal matter and amongst those caught near manure,” illustrating how the fly obtained detrimental pathogens and immediately became the perfect vector for the transmission of disease between species (Hampstead 1914).

While flies were the perfect mechanism by which to spread epidemics during this time period, the flies were largely a result of the urbanization and resulting unsanitary living conditions that prevailed in the 19th and 20th centuries. Overpopulated houses, lack of proper sanitation, among other factors created not only the perfect environment for disease to emerge, but also the perfect conditions for flies, the primary transmitters of disease (Bethnal Green 1925). Large collections of manure in the streets and cess pools allowed flies to expand their population, as they had numerous breeding grounds (Velton). While flies bred in manure, they tended to feed off of many foods similar to those popular in human households, thereby, exposing humans to the pathogens they obtained from manure (Shoreditch 1907).

One of the primary reasons flies were recognized as disease spreading vectors was due to the rise and fall of the fly population. In urban areas, “filth flies” increased with a rise in temperature as did diarrheal diseases, however diarrheal diseases decreased during the winter season as well as with fly control efforts (Graczyk, et al). In another study performed in 1908, specimens were obtained from both diarrhea patients and flies and the conclusion showed the same bacteria strain present in both the patients and flies (Paddington 1908). Many years later, in the Greenberg 1964 study, infected flies were exposed to a common household beverage, and the results showed that 8/10 drinks became contaminated. Furthermore, those drinks were then given to ten volunteers who ingested the drinks. Of the ten volunteers, six became infected with an S-typhimurium bacteria. This study emphasized an example of how flies spread disease in the past and illustrated how detrimental the role of flies can be in the transmission process of disease (Wolff, et al).

The common household fly was known to be the cause of infection in young infants due to their consumption of milk. Doctors during this time believed flies to be the agents of spreading “diarrhoea” among young children. Young children were infected the most with diarrhea due to the high consumption of milk that was contaminated by flies. Flies often contaminated milk before even arriving to its final destination (Deptford 1913). Flies rely on water or milk to survive. When milk or food is left uncovered, flies often land on the surface of the items and contaminate it by their mechanisms of eating, which requires the fly to excrete saliva in order to dissolve the food it is to consume (Hackney 1912). In Europe during the 20th century, summer diarrhoea was said to be the direct cause of contamination of food, especially milk. If a child was breastfed, he or she was less likely to be infected with a disease or have diarrhea, while children that drank milk from a store had a much greater possibility of acquiring such an infection. However, flies can contaminate any children from simply landing on the baby’s mouth (Lambeth 1912). This observation gave an explanation for why babies who did breastfeed could still be infected.

In the burrough of Acton, there was a great concern for the unbelievably high number of deaths in infants. Dr. Newsholme declared that infants who were breastfed were at a lower risk of obtaining infections leading to fatal diarrhea. Reports were constructed and it was observed that among the 64 infants that died, all had been fed some form of artificial milk, with over half being fed cows’ milk (Acton 1911).

Diarrhea in infants and children was more prevalent in the warmer months, and this was thought to be because there are a greater number of flies present. Dr. Nash from the burrough of Acton observed the correlation between the increase in flies and the increase in individuals with diarrhea during the summer months. Dr. Nash revealed that the occurrence of diarrhea follows the life cycle of the common housefly. For example, whenever more flies are present in an area, the amount of individuals infected with diarrhea also increases. Dr. Nash proclaimed that the fly was actually a very dangerous “pathogenic agent” (Acton 1906), capable of transmitting infections across locations. Flies are more abundant in the summer months because the breeding of flies is facilitated by moisture in the air and higher temperatures (Keiding 1986, 304). The flies travel into the homes for not only food, but also during the nights when the temperature drops (Hornsey 1921). As stated before, the milk tended to be contaminated from flies carrying microorganisms upon arrival to the main location. During the transfer from one place to another, the warmer weather in the summer months favored the rapid growth of the bacteria that it was infected with. The enhanced rate of growth of microorganisms in the milk during the summer months posed a great threat to individuals in Europe during this time.

Flies were more prevalent in poorer, less sanitary regions of London. Flies were not as common in the wealthier areas because they were not as filthy and flies thrive in dirtier areas. However, it was observed that infants living in wealthier and more sanitary areas of the city also contracted diarrhea, even though there was no fly problem in the cleaner parts of London. Soon, it was realized that flies were actually contaminating the milk on cattle ranches, and the same milk that was fed to infants. Milk had somehow turned from a “wholesome food to a virulent dangerous poison” through the contamination caused by flies (Deptford 1911). This ignited public health policies that made announcements about the importance to refuse to purchase milk from dairy farms where flies were tolerated (Willesdon 1920).

The mechanism of contamination of milk upon arrival to its destination was unknown until more recently. In Europe, cows were kept in a unit of 400 cubic feet, which was considered very inadequate. The units were kept unclean and had very poor ventilation. The accumulation of cow manure attracted an abundance of flies to the areas. Researchers in Europe did not make the correlation until there was an outbreak of typhoid in Washington DC in 1895. A connection was then made between “flies as a disease vector and the infantile diarrhoea” (Atkins 2012, 42). The increase in cows and horses during the 19th century certainly increased the amount of manure present and led to an increase in the fly population and an increase in “bacteriological flows” (Atkins 2012, 43).

Typhoid and cholera

While children drinking milk had the greatest risk of having an infection leading to diarrhea caused by flies, cholera and typhoid were also associated with flies. Those that lived in the poorer areas were at much greater risk for cholera, due to the decaying animal matter that existed in these areas (Hampstead 1914). Those that were wealthy also would get infected with cholera; however, it did not flourish in the wealthier areas (Islington 1866). The conditions that promoted outbreaks of cholera are typically present in the poorer areas of Europe. While the spread of diseases such as cholera can be attributed to a dirty environment, flies are attracted to these dirtier areas due to the high quantity of decaying food and are responsible for spreading the diseases to other locations.

Typhoid is transmitted to humans from contaminated food or water. Flies are also known to transmit typhoid to individuals as well. Dr. Daniel Jackson constructed a chart revealing the weekly deaths of typhoid in relation to flies (London County Council 1909). The housefly is often called the typhoid fly. Like diarrhea, typhoid can also be spread via cows’ milk. Many doctors urged mothers to breastfeed their children because of the high risk of babies obtaining such infections (Islington 1911). Typhoid caused a plethora of deaths during the Spanish-American War. What exactly was spreading the typhoid among the soldiers was unknown until the conditions of the camps were examined. Researchers concluded it was due to the housefly and now recognized the housefly as a health menace (Cirillo 2006, 52). Flies certainly are capable for transmitting diseases and are still a health menace in some parts of the world today.


Works Cited

Atkins, P. J. Animal cities: beastly urban histories. Farnham Surrey: Ashgate, 2012.

Vincent J. Cirillo. “”Winged Sponges”: Houseflies as Carriers of Typhoid Fever in 19th- and Early 20th-Century Military Camps.” Perspectives in Biology and Medicine 49, no. 1 (2006): 52-63.

Graczyk, Thaddeus K., Ronald Knight, Robert H. Gilman, and Michael R. Cranfield. “The role of non-biting flies in the epidemiology of human infectious diseases.” Microbes and Infection 3, no. 3 (May 11, 2001): 231-35.

Keiding J. The housefly—biology and control. Training and information guide (advanced level). Geneva, World Health Organization, 1986

Velten, Hannah. Beastly London: A History of Animals in the City. Reaktion Books, 2013.

Wolff, H. L., & Van Zijl, W. J. (1969). Houseflies, the availability of water, and diarrhoeal diseases. Bulletin of the World Health Organization, 41(6), 952–959.

Medical Officer of Health Reports

Acton 1911

Acton 1906

Deptford 1913

Hornsey 1921

Lambeth 1912

Islington 1866

Islington 1911

London County Council 1909

Hampstead 1914

Hackney 1912