Flies(苍蝇) are one of the most dangerous enemies of human. They are dangerous because they carry diseases(疾病) , Flies do not care whether the food they eat is dor clean. The food they like best is othe dirtiest that can be found. Then they may finto a house where good clean food is kept and begin to walk all over it and eat it. Soon the good clean food becomes unfit(不合适) for people to eat. Often a person who is preparing fdoes not realize that it is unfit for people to eat, so the food is put on the tand eaten. Those who eat such food become dangerously ill. That's why we must get rid of(消灭) flies.