How Does a Computer Virus Spread?

Computer viruses have come a long way since their first appearance in the 1970's. As pieces of programming code, they're written to perform a specific function. Viruses can spread through email attachments, application programs on your computer, or application programs installed from disk drives.

How Does a Computer Virus Spread?

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In order to spread through a computer system, or network, a virus must be activated; meaning, the file that contains the virus must be opened, or executed in order for the virus to come to life. Once activated, viruses look for certain programs to infect, depending on what they've been programmed to do. The one thing that all viruses seem to do well is duplicate themselves within the programs they find, and that is how they spread.


The effects a virus can have on your computer depends on how its been programmed to spread. Programming can be one of two types: resident and nonresident. Nonresident viruses are made up of two processes -one finds files on your computer to infect, and the other duplicates the virus within the files that have been found.

Resident viruses are made up of one process. It's sole function is to duplicate itself throughout your computer system. To do this, resident viruses are programmed to load into the computer's memory. By doing this, the virus itself is activated every time you turn your computer on. This process allows the virus to remain active and spread to other targeted applications on your system for as long as the computer is on.


There are four types of viruses that can spread through your system.

  1. Program - this type is executed from within specific application programs. The virus spreads to other programs on the computer, or when a copy of an infected program is loaded onto another system.
  2. Boot sector - this type enters your system through a floppy disk, and then infects the hard drive. The result is any time a floppy disk is used, it will become infected.
  3. Macro - this type is written specifically for macro applications on your computer like Microsoft Word, Excel and Outlook. When an infected document is opened within one of these programs, the program itself becomes infected, allowing the virus to spread to any document opened up inside the program.
  4. Email - this type spreads through email attachments. Once an infected attachment is opened, the virus enters your email application. It then duplicates itself by automatically emailing everyone listed in your address book.


Nowadays, computer systems are built to protect themselves against viral infection. The floppy disk drives are quickly becoming a thing of the past, so this point of entry has become less popular. Macro viruses rely on security holes within programs like Microsoft Office, so the newer versions of these programs are less vulnerable, but can still be infected. Email viruses are the ones that are most likely to get inside your system.

The best practice is to not open any email attachments from people you don't know. Email attachments from people you do know should be approached with caution if they're worded strangely, or if you've no reason to expect an attachment from a particular person. And lastly, always keep an updated version of anti-virus software on your computer. A good anti-virus program will not only monitor your system, but will screen incoming emails for suspicious content.

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