There are many bacteria and parasites that  infest the fly, making flies a major factor in spreading many diseases by touching surfaces with their legs or their saliva. After walking on much excrement, flies may carry up to as many as 6 million bacterias on their feet. So be wary of foods that have been touched by a fly! Here's some information on the diseases some flies are known to help spread.
Leishmaniasis Onchocerciasis
 This disease is found in South America, Africa, Indian Subcontinent and Europe.
 It's caused by a parasitic protozoan transmitted by the bite of sand flies.
 Symptoms of this disease usually includes fever, weakness, swollen spleen, and skin sores.
 There is no treatment for this disease; it eats away at your skin.

 Onchocerciasis, also known as River Blindness is an infectious cause of blindness .
 It is carried by a minute nematode worm that is spread bythe Simulium black fly.
  Found in South America and Africa, a bite from this insect can transmit the worm to its victim.
 The drug invermectin has helped stop the progress of the disease.
African Trypanosomiasis Bartonellosis
 This is a sleeping sickness and epidemic caused by a protozan blood parasite Trypanosoma.
 It is trasmitted by the salivary glands of infected Tsetse flies in Africa.
 Symptoms include a boil- like sore at the site of the bite, fever, headaches, and severe illness.
 Treatment should apply in the early stages of the disease by anti-parasitic drugs.		   Found in South America, this disease is caused by the rickettsia organism transmitted by the bite of a Sand Fly.
 Victims are usually exhausted from anemia, and experience a high fever and wart-like eruptions on the skin.
 Treatment is available.
Myiasis Typhoid
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 This disease occurs mostly on animals such as dogs and sheep and cows, but sometimes it may occur on humans, more frequently carried by the Cheese Skipper fly.
 It is trasmitted by a fly that lays its eggs on the skin of another organism. The larvae can burrow into the skin or penetrate itself in open wounds.
 Symptoms include violent abdominal pain, nausea, vomiting, and diarrhea with bloody discharge.
 There is currently no treatment for this disease (on humans, that is; on animals insecticides are used), except to either let the larvae grow and leave at its own accord, or to remove it by enticing it.
 Flies rummaging around excrement may come across bacteria known as Salmonella Typhi, which may come from a person shedding it.
 This disease is a type of fever. Symptoms include a body temperature of as high as 103° to 104° F (39° to 40° C), weakeness, stomach pains, headaches, and/or loss of appetite.
 This disease can be found all over the world except in industrial countries such as United States, Western Europe, and Japan.
  Treated with the drug chloromycetin, or ampicillin for those infected with bacteria that is immuned chloromycetin.
Dysentery Leprosy
 This is a chronic disease that affects the large intestine in humans.
 The parasite Entamoeba histolytica is the cause of this disease. This particular parasite can be found in uncooked meats, and may be transported by flies.
 This disease is characteristic of sever diarrhea and severe stomache cramps.
 Treatment with drugs containing metronidazole or ementine is recommended.		   Leprosy, also known as Hansen's Disease, is a chronic disease that affects mainly the skin.
  It is caused by the bacillus Mycobacterium leprae, which may be carried by flies from rotted foods.
  An early symptom is anesthesia (or the numbness) of a patch of skin. Some muscles may be paralyzed. Because of the numbness of some nerves, injuries to the area are not noticed.
 This disease is generally rare nowadays, but that doesn't mean it doesn't exist. A vaccine for leprosy is currently being developed.