Ebola may not be a deadly disease for everyone，after scientists discovered that some people are likely to be naturally immune（有免疫力酌 ）to the virus．

A study in mice showed that genetic variations （基因变异） govern how ill victims will get after contracting the disease．Some completely resist the disease，while others have a slight effect．However，many still suffer from bleeding，organ failure and shock．

The rese．arch was conducted in a highly secure，top-level bio lab in Montana，US．Researchers found that all mice lost weight in the first few days after infection．However，nearly one in five of the mice not only survived，but also fully regained their lost weight within two weeks．

After a fortnight they had no evidence of the disease and their livers looked normal．One

in nine of the mice were partly resistant and less than half of these died．But seventy percent of the mice had a greater than 50percent death rate．

The authors believe that those who have survived during the recent outbreak in West Africa may have had natural immunity．"Our data suggests that genetic factors play a significant role in disease outcome，"said co-author Dr Michael Katze，of Washington University's Department of Microbiology．

"We hope that medical researchers will be ahle to rapidly apply these findings to candidate therapeutics and vaccines（疗法和疫苗）．"

28．According to the passage，the degree of the victims'disease is determined by___．

A．their weight B．genetic factors

C．natural immunity D．their resistance

29．What is the symptom of Ebola in the study'？___

A．All the mice lost weight．

B．The mice all began to bleed．

C．All the mice suffered organ failure．

D．The mice all gained weight．

30．Which of the follou'ings is true in the research？___

A．Seventy percent of the mice died．

B．About twenty percent of the mice survived．

C．The