Computer Viruses are small software programs designed to ruin computer's functionality without any prior knowledge or permission of the user. An infected computer can affect another computer if its host is taken to the uninfected computer. There are several modes of computer virus transmission. It can easily be sent over the Internet or by carrying it on a removable medium such as a floppy disk, CD, or USB drive. It can stop to infect your computer by using anti-virus support software program.

Viruses damage computers by deleting files, infecting programs, or reformatting the hard disk. Some of them can just replicate and strengthen their presence by presenting text, video, audio messages. Such viruses often cause erratic behavior and lead to system crashes. Examples include opening a document infected with a macro virus, using an infected disk, network drive, and other media. As long as the virus remains active on the computer, it has the ability to copy itself to other files or accessible disks.

The more a virus replicates unnoticed, the more it gets successful in its aim. Computer viruses generally follow two methods to avoid recognition:

• By encrypting their code to escape detection.
• By disabling the options to view macros.

There are some old viruses which ensure the last modified date of a host file to be the same as when the file was infected. However, such an approach cannot deceive anti-virus software created to catch and remove these craps.

Some viruses have ability to damage files without increasing their sizes or damaging the files by overwriting unused spaces of executable files. They are better recognized as cavity viruses. Computer viruses such as the CIH virus, or Chernobyl Virus, infect Portable Executable Virus.

Some viruses prevent detection by ending the tasks associated with antivirus software before it can detect them. Anti-virus and PC security products are required to be updated or replaced to stay in step with increasing advancement in computers and operating systems.