传染病的特征是
A．有传染性和流行性，死亡率较高
B．有传染性，但不会死亡。
C．有的由病原体引起，有的由体虚引起
D．都由病原体引起，有传染性和流行性
答案：D
题目分析：传染病是有病原体引起的，能在生物之间传播的疾病．传染病一般有传染源、传播途径和易感人群这三个基本环节，而传染源是指能够散播病原体的人或动物；传播途径是指病原体离开传染源到达健康人所经过的途径，易感人群是指对某些传染病缺乏免疫力而容易感染该病的人群．同时具备了以上三个环节，就可以传染和流行．
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| For the first nineteen months, Helen Keller was like other pretty, happy babies in every way. Then a sudden illness destroyed her sight and hearing. 1 she could not hear what other people were saying, the child couldn't 2 to speak. For the next five years, she lived in a world of darkness 3 sounds and words. The person who changed Helen's world was her teacher, Anne Sullivan, who 4 her life in 1887. Miss Sullivan had agreed 5 a blind child who acted like a young animal, rushing wildly around, throwing and hitting anyone whom she could 6 . Who could believe that 7 child could be taught? But Anne had been blind during part of her childhood and had learned to read Braille, 8 is a system of raised dots that can be felt by the fingers of a blind person. She had learned 9 again only after several operations, and she 10 forgot the experience of 11 . Miss Sullivan couldn't teach Helen to speak 12 some other important things 13 . The little girl had to learn to control her actions and feelings. She had always been able to get what she wanted 14 force. The teacher had to 15 her learn that she could not always do 16 she wished to do. Miss Sullivan's battle began. Sometimes there was real fighting 17 the wild child and the strong teacher. 18 , however, the battle was won by Miss Sullivan, who had succeeded 19 Helen that she loved her and wanted to help her. The child and the teacher became friends. They 20 be friends until the teacher's death, fifty years later. | ||||
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