高考英语冲刺阅读理解训练题（五十二）_高考复习指南

高考英语冲刺阅读理解训练题（五十二）

　　There are three kinds of influenza virus, known as A, B and C. Influenza C virus is relatively stable and causes mild infections that do not spread far through the population. The A and B types are unstable, and are responsible for the epidemics that cause frequent concern. Following any virus attack, the human body builds up antibodies which can be changed into immunity（免疫力）

　　to that strain of virus but a virus with the ability to change its character is able to bypass this protection. Variability is less developed in the influenza B virus, which affects only human beings. As influenza B virus may cause a widespread epidemic but will have little effect if introduced into the same community soon afterwards, since nearly everyone will have built up antibodies and will be immune. The influenza A virus, which affects animals also, is extremely unstable and is responsible for some of the worst outbreaks of the disease, such as the world epidemic, of 1918&1919, when about half the world’s population were infected and about twenty million people died, some from pneumonia caused by the virus itself and some from secondary complication caused by bacteria. Accurate prediction is difficult because of the complication of the factors. A particular virus may be related to one to which some of the population have partial involved immunity. The extent to which it will spread will depend on factors such as its own strength, or virulence, the ease with which it can be transmitted and the strength of the opposition in encounters. Scientists, however, have a reliable general picture of the world situation.