Although we already know a great deal about influenza, and although the World Health Organizations is constantly collecting detailed information from its chain of influenza 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 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 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 it encounters. Scientists, however, have a reliable general picture of the world situation.

虽然我们对流感已经了解了很多，虽然世界卫生组织不断从其遍布于世界各地的参考实验室那里收集详细的资料，对研究传染性疾病的流行病学家来说，预测下一次流感疫情爆发的时间和地点及其严重性依然极其困难。
流感病毒有三种，即A、B和C型。Ｃ型流感病毒相对稳定，只引起轻微的感染，不扩散并感染大量人口。A型和B型是不稳定的，这两种病毒是导致流感疫情，引来频繁关注的罪魁祸首。在任何病毒攻击之后，人体积聚的抗体可以改变自身并对该株病毒形成抵抗力，但病毒也拥有改变自身性质的能力并绕过这种保护。在只影响人类的Ｂ型流感病毒中，变异的情况不甚严重。因此Ｂ型流感病毒可能会造成大面积疫情，但在不久之后引入同一个社区就没有什么效果，因为几乎每个人都已经建立了抗体并形成了免疫。而同时影响动物的A型流感病毒则极不稳定，并对一些最严重的疫情暴发负有责任，如1918年和1919年世界范围内的疫情，当时全世界大约有一半的人口受到感染，约2千万人死亡，一些死于病毒本身引起的肺炎，另一些则死于该病菌引起的并发症。准确的预测因为其复杂的因素而变得非常困难。一种病毒可能从另一种某些人口可以部分免疫的病毒衍变而来。其蔓延的程度将取决于许多因素，如其自身的强度或毒性，是否容易传播以及它遇到的抵抗的强度。然而，科学家对流感病毒在世界范围内的大致形势有着可靠的了解。