Once attached to me host Program，the viruses then look for other programs to "infect"．In this way， he virus can spread quickly throughout a hard disk or an entire organization when it infects a LAN or a multi-user system． At some point，determined by how the virus was programmed the virus attacks．The timing of the attack can be linked to a number of situations，including a certain time or date， the presence of a particular file，the security privilege level of the user，and the number of times a file is used．Likewise，the mode of attack varies．So－called "benign" viruses might simply display a message，like the one that infected IBM’s main computer system last Christmas with a season’s GREeting．Malignant viruses are designed to damage the system．The attack is to wipe out data， to delete flies， or to format the hard disk.
What Kind of Viruses Are There？
There are four main types of viruses: shell, intrusive, operating system and source ode．
Shell viruses wrap themselves around a host program and don’t modify the original program．Shell programs are easy to write，which is why about half of viruses are of this type．
Intrusive viruses Invade an existing program and actually insert a portion of themselves into the host program．Intrusive viruses are hard to write and very difficult to remove without damaging the host file.
Shell and intrusive viruses most commonly attack executable program flies－those with a．corn or．exe extension， although data flies are also at some risk．
Operating system viruses work by replacing parts of the operating system with their own logic．It is very difficult to write operating system viruses and these viruses have the ability once booted up， to take total control of your System．FOF example，some operating system viruses have hidden large amounts of attack logic in falsely marked bad disk sectors．
Source code viruses are intrusive programs and they are also inserted into a source program such as those written in Pascal prior to the program being complied．There are the least－common viruses because they are not only hard to write，but also have a limited number of hosts compared to the other types．
Be Wary of the Second Network Viruses
Do you believe it？ Network viruses can steal money！So far Internet has become the main channel through which the computer viruses spread．Look，here come the second network computer viruses．Even without "snatching" information from the network your computer can be infected by the second network computer viruses， which are hidden in some machines on the network． Your computer is， so to speak， in danger once being connected to the network to browse．
The virus that can steal your money belongs to a kind of the second network viruses. It was designed and put in some machines on the network． When your computer is linked to one of these machines，the virus will invade your hard disk and search whether Intuit Quicken， an accounting software， is installed． One of this accounting software’s functions is to transfer accounts automatically. Once infected， your money will be transferred to an additional account opened by the virus program without anybody knowing it．
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计算机病毒
计算机病毒是八十年代计算机飞速发展带来的结果，计算机病毒这一名词起因于在计算机上传染的有害程序与生物学中病毒的相似性，该名词起源于1977年出版的、由 Thomas J． Ryan写的一本美国科幻小说《p-1的青春》。人体病毒入侵活的细胞后，就把它转变成制造病毒的工厂。而计算机病毒是一些小程序，它们把自己的一个副本附加到另一个程序上面进行复制。
病毒程序一旦附加到一个主程序上，就开始寻找其他可以进行“感染”的程序。这样，病毒就很快地布满整个硬盘，如果病毒感染了一个局域网或者一个多用户系统，那么就在整个组织内进行扩散。只有在某个点上病毒程序才会发作（这是由病毒程序的设计所决定的）。攻击的时间与多种情况有关，包括：某个时间或日期、特定用户识别符的出现、用户的安全保密等级和一个文件使用的次数。同样，攻击的方式也是各种各样的。所谓的“良性”病毒可能只是简单地显示一个消息，如在去年圣诞节时，受感染的IBM公司的主要计算机系统显示节日问候。恶性病毒被设计出来破坏系统。常见的攻击方式是消除数据、删除文件或者格式化硬盘。
病毒程序有哪几种？
有四种类型的病毒程序：外壳型、入侵型、操作系统型和源码型。
外壳型病毒将它们自己包裹在主程序的四周，对源程序不作修改。外壳型病毒容易编写，这也正是约有一半病毒程序是这种类型的原因。
入侵型病毒侵入已有的程序，实际上是把病毒程序的一部分插入到主程序中。入侵型病毒难以编写，在去除它们时常常会破坏主文件。
外壳型和入侵型病毒通常攻击可执行文件，即带有.COM或．EXE扩展名的文件。当然数据文件也有受攻击的危险。
操作系统型病毒发作时用自己的逻辑代替部分操作系统。这些病毒程序的编写非常困难，它们一旦得手就有能力控制整个系统。例如，一些操作系统型病毒把自身的逻辑隐藏在那些标识“坏”的磁盘扇区中。
源码型病毒是入侵程序，它们在程序被编译之前插入到诸如用PASCAL编写的源程序中，它们是最少见的病毒程序，因为它们不仅编写困难，而且与其他类型的病毒相比，受它们破坏的主程序数目也有限。
小心第二代网络病毒
相信吗？网络病毒会偷钱！ 目前，因特网已成为电脑病毒传播的主要渠道。瞧，第二代网络病毒又来了。无需从网上“抓取”资料，你的机器就会感染上第二代网络病毒，这种病毒匿藏于网络上的一些机器中。一旦连接上网测览，你的机器就可以说是处于危险之中了。
会偷钱的病毒属第二代网络病毒。这种病毒被设计出来之后，就放置在网上的一些机器中。当你的计算机连接到这样的机器上时，病毒就会入侵到其硬盘内，并搜寻其中是否装有“Intuit Quicken”理财软件。此理财软件的功能之一就是自动转帐。一旦感染第二代病毒，你的钱就会神不知鬼不觉地被病毒程序转到它所建立的其他帐户上了。
