Computer virus is a software program which is able to copy itself from an infected item to another infected one. (This item may be a program file, document or computer, etc.)
There are various ways for a virus to spread to another item and cause harm to the infected system. Virus itself is a type of program designed to serve malicious purposes. Like real-world diseases, there are cases which require long-time research to find the treatment, and there are diseases which result in dire consequences. Therefore, "prevention is better than cure" is the basic tactic which is also applicable to computer virus.
How can computer virus spread?
Virus can infect your computer via emails and files downloaded from the Internet or copied from USB drives or other computers. Hackers can also take advantage of software vulnerabilities to remotely install virus on your computer without your awareness.
Email is the principal and the most common way to propagate virus. Virus is able to harvest emails stored in a computer and send forged emails with luring subjects. These emails are often attached with a malicious file. Some kinds of virus even quote a part of the victim's email content to make it more authentic to users. By this way, the virus is able to propagate itself by geometric progression.
Your computer can also be infected with virus when you run a program downloaded from the Internet or copied from another infected machine. The reason is that this program has been infected with virus before or the program itself is a virus masquerading as a legitimate program. Thus, when you run this program you activate the virus at the same time.
There are also a great number of viruses infecting computers via binary codes installed in malicious websites, typically virus spreading via Yahoo! Messenger, Windows Messenger, etc. Such website owners attempt to entice users to visit their sites, and then the malicious code will be executed to infect virus to their machines.
Softwares and even operating system always contain potential flaws which may later be exploited to distribute malware. Your computer may be infected even when you do not open attached file in unknown email or run a suspicious program since virus is able to infect via security hole of your vulnerably software.
How can virus harm your computer?
Virus is written, more or less, to serve malicious purposes.
Viruses sometimes generate annoying messages or produce a lot of copies to flaunt their presence. However, they are also able to seriously destroy your data, disk drive and your system. They can take advantage of affected machines to distribute advertisements or collect your friends' email addresses. Viruses may be used to steal your bank or email account or your personal information. Once infected with virus, your machine can become the hacker's tool to hijack other systems or the network system you currently use. Sometimes you are the target of the predator, but in many cases, you may become the hacker's assistant to attack other systems.