Just as human viruses invade a living cell and then turn it into a factory for manufacturing
viruses, computer viruses are small program that replicate by attaching a copy of themselves to
another program. Once attached to the host program, the virus then lock for other programs to
"infect". In this way, the virus can spread quickly throughout a hard disk or an entire organization if

it infects a LAN (Local Area Network) or a multi-users system.
[2] Skillfully written virus can infect and multiply for weeks or months without being detected.
During that time, system backups duplicate the viruses, or copies of data or programs made and
passed to other systems to infect. At some pointˆdetermined by how the virus was programmedˆ
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 user ID; the use or presence of a particular file; the
security privilege level of the user; and the number of times of a file is used.
Likewise, the mode of attack varies, so-called "being" 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, on the other hand, are designed to damage your system. One common
attack is to wipe out data, to delete files, or to perform a format of disk.
There are four main types of viruses: shell, intrusive, operating system, and source code.
Shell viruses wrap themselves around a host and do not modify the original program.
Shell program are easy to write, which is why about half of all viruses are of this type. In
addition, shell viruses are easy for programs like Data Physician to remove.
Intrusive viruses invade an existing program and actually insert a portion of themselves
into the host program. Intrusive viruses are hard to write and difficult to remove without
damaging the host file.
Shell and intrusive viruses most commonly attack executable program fileˆthose with.
COM or. EXE extensionˆalthough data are also at some risk.
Operating system viruses work by replacing parts of operating system with their own
logic. [4]Very difficult to write, these viruses have the ability, once booted up, to take
total control of your system. According to Digital Dispatch, known versions of operating
system viruses have hidden large amounts of attack logic in falsely marked bad disk
sectors. Others install RAM-resident programs or device drivers to perform infection or
attack functions invisibly from memory.
[5]Source code viruses are intrusive programs that are inserted into a source program
such
as those written in Pascal prior to the program being compiled. These 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.
New computer viruses are written all the time, and it's important to understand how your
system can be exposed to them and what can do to protect your computer. Follow the suggestions
listed below to substantially decrease the danger of infecting your computer system with a
potentially dangerous computer virus.
Be very cautious about inserting disks from unknown sources into your computer.
Always scan the disk's files before operating any of them.
Only download Internet files from reputable sites.

Do not open e-mail attachments (especially executable files) from strangers.
Purchase, install, and use an anti-virus software program. The program you choose must
provide three functions:
Detection.
Prevention.
Removal.
As new viruses are created everyday, upgrade your anti-virus software regularly.

1, invade  [in'veid]
vt. 侵略；侵袭；侵扰；涌入
vi. 侵略；侵入；侵袭；侵犯

2, replicate  ['replikit, 'replikeit]
vt. 复制；折叠
vi. 重复；折转
adj. 复制的；折叠的
n. 复制品；八音阶间隔的反覆音

3, wipe  [waip]
n. 擦拭,用力打
v. 擦,消除,拭去