A computer virus is a program that installs and multiplies itself in a computer—usually without the knowledge or consent of the user. It belongs to a larger group called malware or malicious software. The term virus is erroneously used as synonyms to adware, spyware, worms and Trojan horses.
Only viruses have the capability of reproduction.

Types of Infection Methods
Viruses reproduce and spread by attaching themselves to executable files (usually .exe in the Windows operating system (OS)). Therefore, they cannot spread by themselves. The .exe files should be run first before the virus can multiply.

Other viruses embed themselves in boot sectors of hard drives, floppy diskettes and thumb drives. When a computer is booted through the drive or portable device, the virus code is activated.
           
A computer virus cannot attach itself on “pure data” files, which include music (mp3, wav), videos (mpeg, avi) and images (jpeg, bmp). However, they can infect Word documents and Excel spreadsheets that use macros. These are called macro-viruses. Notepad (.txt) files are harmless; however, those that contain Java can also be infected.

Whenever the infected program is executed, the virus code works to infect other programs in the computer. Many viruses also use the network connection to search and infect files in other PCs. Thus, its spreading becomes exponential.

Effects to the Computer
  A computer virus may simply be annoying—creating folders and replicating itself. However, some may drastically affect the computer’s performance.

Note that viruses are also programs, which follow the codes written by the creator. Therefore, they cause extra load to the computer’s memory and processor. This is why CPU meters rise to 90-100% even if there is no “real program” being run.

Some can deliberately delete files. Some attach too well, practically destroying a file and rendering it useless. Sometimes, a file cannot be recovered even with an antivirus- giving the owner no choice but to delete it.Other than software damage, a computer virus cannot cause physical damage like frying of processors and hard drives.
           

Avoiding Infection    
The primary step is to always update your Operating System and antivirus software. More potent viruses are being released every day in the internet. A high security OS with antivirus is your best line of defense. A good OS will require user’s permission for every installation and registry modification
           
Next, do not just run any downloaded program. Scan it first. It may be a virus-infected software, or it may be a spyware or a Trojan horse. Other malware do not reproduce, but their effects are equally devastating. Some might send your personal information (including PINs and passwords) to third parties.
           
Some viruses may use messengers in sending links and emails. Do not just click the links on your messengers. Make sure that they are really sent by your friends. Scan emails and documents that might be using Java or macros.
           
If you use USB drives, always scan them. Public computers like in shops and schools are usually nesting grounds of malware. Therefore, it is unavoidable that some may jump to your thumb drive files. Scanning with a licensed and updated antivirus will take care of them.