The diseases that flies can spread have been well documented though not always understood. In medieaval times the spontaneous generation of maggots upon rotten meat was not associated with flies. It was not until the seventeenth century and the advancement of the microscope that it was determined that living organisms could only arise through spontaneous generation if they arose only from other living organisms (biogenesis).
In 1668 the Italian physician Francisco Redi sought to demonstrate that meat does not spontaneously produce maggots. In one of the first ever controlled experiments Redi filled six jars with decaying meat. Three were left open and the remaining jars were left open. When maggots developed on the meat in the open jars, Redi believed he had demonstrated that spontaneous generation did not occur. However, supporters of the notion of spontaneous generation claimed that it was the lack of fresh air rather than the absence of flies which had prevented maggots from appearing on the meat.
Therefore, Redi undertook a second experiment, in which he covered the tops of three of the jars with muslin instead of sealing them. Once again, maggots failed to appear on the meat in the covered jars, but did appear on the meat in the open jars, where flies were able to lay their eggs.
More modern experiments have demonstrated that flies walking over culture media in sterile dishes have resulted in the growth of over 100 bacterial and fungal colonies from bacteria and fungal spores which the fly deposited. Many germs are harmless but others can be a vector for serious disease such as typhoid, cholera, ophthalmia and parasitic worms.
Researchers from the Department of Microbiology, All India Institute of Medical Sciences, New Delhi examined how flies could spread disease in the hospital environment and concluded that since pathogenic microorganisms are widespread in the hospital environment, there is abundant opportunity for flies to become contaminated and, in turn, to contaminate the patient environment.
The greatest danger arises when flies are allowed to alight and feed on the faeces of a person suffering from intestinal disease such as typhoid, cholera or polio. Germs or spores of these diseases may thus be carried by the flies to human food. When this is eaten the bacteria can multiply inside the body and so infect the consumer with the disease. Flies are thought to be responsible for the spread of such diseases as those already mentioned and also diarrhoea, anthrax, eye inflammation and possibly tuberculosis.
Today there is a pretty clear disease pathway: flies land on bacterially infected/food/feces/meat then land on your food spreading bacteria, then vomit and defecate on it. Then you eat it. Add open latrines into the mix (as it were) and you multiply the disease risk by magnitudes. Flies are a nuisance in the developed world, a killer in the developing world. Good sanitation is key to fly control.
Food for human consumption must not be stored or enable flies to settle on it. The numbers of houseflies should be kept as low as possible by removing left over food.
Many thanks to Biology Resources http://www.biology-resources.com/housefly-01.html
Spontaneous Generation – Meat, Life, Pasteur, and Flasks – JRank Articles http://science.jrank.org/pages/6408/Spontaneous-Generation.html#ixzz264qY8Dtd