What are computer viruses?
According to Fred Cohen’s well-known definition, a computer virus is a computer program that can infect other computer programs by modifying them in such a way as to include a (possibly evolved) copy of itself. Note that a program does not have to perform outright damage (such as deleting or corrupting files) in order to be called a “virus”. However, Cohen uses the terms within his definition (e.g. “program” and “modify”) a bit differently from the way most anti-virus researchers use them, and classifies as viruses some things which most of us would not consider viruses.
Computer viruses are bits of code that damage or erase information, files, or software programs in your computer, much like viruses that infect humans, computer viruses can spread, and your computer can catch a virus when you download an infected file from the Internet or copy an infected file from a diskette. Once the viruses is embedded into your computer’s files, it can immediately start to damage or destroy information, or it can wait for a particular date or event to trigger its activity.
What are the main types of viruses?


什么是計算機病毒？
按照Fred Cohen的廣為流傳的定義，計算機病毒是一種侵入其他計算機程序中的計算機程序，他通過修改其他的程序從而將（也可能是自身的變形）的復(fù)制品嵌入其中。注意一個程序之所以成為“病毒”，并非一定要起徹底的破壞作用（如刪除或毀壞文件）。然而，Cohen在他的定義（即“程序”和“修改”）中使用的“病毒”這個術(shù)語與大多數(shù)反病毒研究人員使用的“病毒”術(shù)語有些差別，他把一些我們多數(shù)人認為不是病毒的東西也歸類為病毒。
計算機病毒是一些能破壞或刪除計算機中的信息、文件或程序的代碼。正如感染人體的病毒一樣，計算機病毒能夠擴散。當你的計算機從互聯(lián)網(wǎng)上下載一個被感染的文件，或者從磁盤上復(fù)制一個被感染的文件時，你的計算機就會染上病毒。而一旦病毒進入到你的計算機文件中，它就能馬上破壞或摧毀其中的信息，或者等到某個特殊的日期或事件來臨時才觸發(fā)其破壞活動。