What is a virus?
Simply defined, a virus is a computer program that is designed to replicate itself by copying itself into the other programs stored in a computer. Many viruses, however, are additionally configured to harm your computer by altering the way it works. Depending on the virus, these alterations may be as benign as displaying a message on a certain date, or as destructive as erasing your hard drive.
When an infected file is executed, or the computer is started from an infected disk, the virus itself is executed. Often, it lurks in memory, waiting to infect the next program that is run, or the next disk that is accessed. In addition, many viruses also perform a trigger event, such as displaying a message on a certain date, or deleting files after the infected program is run a certain number of times.
Viruses spread differently depending on what type they are. Here is a breakdown of the most common viruses:
Boot sector viruses append themselves to floppy disks. If you leave an infected disk in your floppy drive when you turn your system on or reboot it, the virus copies itself onto the boot sector of your hard drive. You cannot get this virus from sharing files or by executing a program - only from an infected disk.
Program sector viruses fasten themselves to other programs. Most will piggyback on EXE or COM files, but they can infect any file that your computer runs when it launches a program.
Macro viruses affect the template used to create documents or spreadsheets, thereby infecting every document or spreadsheet opened with the program.
Email viruses can infect your computer before you even open an email. For example, a virus named "Bubble Boy," which targets Outlook and Outlook Express, can be launched even if it only opens up in the preview window. Just highlighting the subject line in the preview window activates the code. It starts sending messages to everyone in your address book in an attempt to overload and crash the email server.
Other questions about viruses: