For example, they usually find it difficult to jump between species, because they would have to change their structure. But if large numbers of a host – say, birds – encounter a great number of people, eventually the virus will find a way to prosper in a new type of cell.
有时病毒的特点对我们有利。比如，它们要跨越不同物种交叉感染就很困难，因为要那样做它们就必须改变自身的结构。但如果数量众多的宿主—比如鸟类—遭遇了众多的人类，那么病毒最终会找到在新型的细胞中繁殖的方法。