北京理工大学网络教育2013年春学籍入学考试：英语6

     In the summer of 1897 an English doctor， Ronald Ross， worked in a field hospital in India. Ross set out to find the cause of malaria （疟疾）。 Ross observed that patients in the field hospital who did not have malaria were more likely to develop the deadly disease in the open wards （病房） than in wards with closed windows or screens. Ross suggested a hypothesis （假设）： mosquitoes （蚊子） in the open wards might be spreading the disease from patients with malaria to patients who did not have the disease. Ross predicted （预言） that if mosquitoes were spreading malaria （hypothesis） ， then mosquitoes that had bitten malaria patients and sucked up some of their blood should have picked up the parasite （寄生虫） （prediction） ， which is always present in the blood of malaria victims （受害人）。 Ross also predicted that parasites should be alive within the mosquito. Somehow the parasites make their way from the mosquito's stomach to its saliva （唾液） so that the parasites are transferred （迁移） with the mosquito's saliva to the next person bitten. So if a person is bitten by a malaria-carrying mosquito， that person will receive some of the parasite in the saliva left behind by the mosquito. Ross looked for living malaria parasites in mosquitoes that had bitten malaria patients. He carefully dissected （切开） the mosquito's stomach and found the live parasites. Ross carried out a control experiment， in which the condition suspected （怀疑） to cause this is compared to the same situation without the suspected condition （ a control group）， Nothing else is changed in any way. In Ross's experiment， the suspected condition was mosquitoes feeding on malaria victims. As a control， Ross checked mosquitoes that had not bitten someone with the disease to see if they also contained （包含） the parasite. Gathering mosquitoes that had not yet fed， he allowed them to feed on malaria-free blood， and then he examined them. Their stomachs and saliva lacked （缺乏） the parasite. The control group of mosquitoes did not contain malaria parasite. The experiments proved， that Ross's hypothesis was correct. Ross's theory that malaria is transferred by mosquitoes carrying it from one person to another was in important milestone （里程碑） in medicine. Finding the cause of malaria is one of the greatest medical advances of all time.

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