For a virus or similar program to have any great impact it needs to be able to spread from one machine to another. They are specifically designed to get access to parts of your computer system that allow for communication with other machines. Below is a discussion of some of the most common methods.

Boot Infectors

Every disk (hard disk, floppy, CD, DVD) contains a boot sector whether it is a bootable disk or not. When a computer is turned on, it looks for boot information. If the computer finds a disk with boot information, it reads that information and uses it to properly start the computer. If for some reason that boot information is infected with a virus, the virus is activated and possibly transferred to the computer's hard drive (if the infection was on a CD for example).

Once the boot code on the hard drive is infected the virus will be loaded into your computer's memory every time you start your computer. From memory the boot virus can travel to any and every disk that is put into your computer. This is how the infection spreads.

Most boot viruses could be on a system for a long time without causing problems, simply existing there to spread themselves. Often such viruses are designed to activate their bad behaviour on a specific date (Halloween for example). There are some nasty boot viruses that will destroy the boot information or force a complete format of the hard drive immediately after they get into a computer.

Program Infectors

When an infected application is run the virus activates and is loaded into memory. While the virus is in memory any new program file that you run can become infected. This means that there will be increasingly more applications on your system that are infected. Multiple infections are very common and will certainly cause system problems.

Program files may function without any problems for some time but eventually they will have problems or multiple infections brings the entire computer system down. The data the program produces may be a first sign of infection such as saving files without proper names, or with incorrect/incomplete data being saved.

Viruses of this type are often designed to seek out programs that are used to share information between users/computers such as email applications, screen savers, office document Macros, and self-extracting compressed files.

Through e-mail attachments

Many of the most dangerous viruses are primarily spread through e-mail attachments - files that can be sent along with an e-mail message. In such cases, the user of an infected computer unknowingly attaches an infected file to an email message, and then sends the email to a friend or colleague. When the email is received, the virus is launched when the file attachment is opened, thus infecting a new computer. Email messages with animations, automated greeting cards, jokes, photographs, spreadsheet and document files, all have been documented to contain virus files.

More and more frequently these days mass mailing email worms/viruses are being released. These attacks AUTOMATICALLY scan your computer's files for any email address it can find and then uses your email application to AUTOMATICALLY send infected messages to any email address found in any file on your computer (not just email message files).

It is important to be cognizant of the emails that you open to make sure that they do not contain harmful viruses. Even emails from family and friends could have a virus, if that person's computer is infected. This is where an anti-virus software would be really helpful in detecting if there is a virus in any incoming messages.