Although we already know a great deal about flu, and although the World Health Organizations is constantly collecting detailed information from its chain of flu reference laboratories throughout the world, it is extremely difficult for epidemiologists, who study infectious diseases, to predict when and where the next flu epidemic will occur, and how serious it will be.   There are three kinds of flu virus, known as A, B and C. flu C virus is  ly 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 to that type of virus but a virus with the ability to change its character is able to bypass this protection. Variability  is less developed in the flu B virus, which affects only human beings. As flu 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 flu 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 it encounters  . Scientists, however, have a reliable general picture of the world situation. Which of the following is NOT mentioned as a factor contributing to the extent to which a virus spreads?
The strength of the virus. The ease with which the virus can be transmitted. The strength of the opposition. The immunity the virus can bring about.