Not all heroes are to be found in the army. In fact, many of them have been found in almost every field.
Early this century, a group of doctors were trying to discover the cause of the disease called yellow fever .At that time it was one of the worst diseases known. Thousands of people were dying of yellow fever every year. It had been suggested that the disease was caused by the bite of the certain mosquito(蚊子), That is , it was thought that a certain kind of mosquito would bite a person who was ill with yellow fever and then, flying else where, bite another person and give him the disease.
To prove whether or not this was true, one of the doctors, Dr. James Carroll, allowed such a mosquito to bite himself after it had bitten a person with yellow fever. He knew that the bite of the mosquito might cause his death.
Dr. Carroll became very ill, but he gained his health from the disease in the end .The risk this hero took, however , helped to save the lives of many millions of people.