Although we already know a great deal about influenza, it is difficult for epidemiologists (流行病学家), who study infectious disease, to (36) when and where the next flu epidemic will occur, and how (37) it will be. There are three kinds of influenza virus, known as A, B and C. Influenza C virus is relatively stable and causes mild (38) that do not spread far through the population. The A and B types are (39) , and are responsible for the epidemics that cause frequent concern. Following any virus attack, the human body builds up antibodies which confer (给予) immunity to that strain of virus, but a virus with the capacity to change its character is able to bypass this (40) . Variability is less developed in the influenza B virus, which affects only human beings. An influenza B virus may cause a widespread but will have little effect if (41) into the same community soon afterwards, since nearly everyone will have built up antibodies and will be immune. The influenza A virus, which (42) affects animals, is extremely unstable and is responsible for some of the worst outbreaks of the disease, such as the unparalleled pandemic, or world epidemic. Accurate prediction is different because of the complication of the (43) . A particular virus may be related to one to which some of the population have (44) involved immunity. The extent to which it will depend on factors such as its own strength, or virulence, the ease with which it can be (45) and the strength of the opposition it encounters. Scientists, however, have a reliable general picture of the world situation, according to which we may take some precautions. A. partially E. generally I. epidemics M. unstable B. guess F. protection J. also N. infections C. factors G. transmitted K. attacks O. predict D. equally H. severe L. introduced