Flies like to live on feces and decomposed organic matter.
In various decomposed dirty places, there are large numbers of bacteria of various colors. Flies stay in these dirty places, eat these dirty things, and inevitably get infected with many bacteria. According to some scientists’ research, the number of bacteria carried by a fly often ranges from several hundred thousand to over five hundred million.
Many of the bacteria infecting the fly’s body mainly hide in the digestive tract. Most of these bacteria are harmful to humans, such as Salmonella, Shigella, and other pathogens. However, these bacteria do not harm the fly. Some pathogenic microorganisms, although they can survive and reproduce in the bodies of vector insects, are harmless to the insects themselves. This is due to an adaptation formed between pathogenic microorganisms and vector insects during long-term evolutionary history. According to experiments, many bacteria harmful to humans only survive in the fly’s digestive tract for five or six days, with some dying and some being expelled with feces. Therefore, although flies specialize in dirty places and carry many bacteria on their bodies, these bacteria cannot survive inside them, so they do not get sick.