In many cases, any type of malicious program or code that affects or impairs a computer's functionality is referred to as a "virus". The term "virus" is often used as an umbrella term that covers everything from worms to adware. However, technically a computer virus has a more specific definition. Here we will go over the primary differences between computer viruses, worms, and Trojans.
The three main types of computer viruses are file infector viruses, boot sector viruses, and macro viruses. What sets them apart?
File Infector Viruses
Viruses which infect other files or programs on your system are considered file infector viruses. Once the original "host" program is executed, the virus may dwell within your system's memory (RAM) and infect applications as they are opened. The virus may also simply lay dormant inside a specific program, and then infect another program or file every time its host program is run. More advanced file infectors do not actually change the program itself, but rather they alter the route that the computer uses to open files. In this case, the virus is actually executed before the original program. If a file or program infected with the virus is passed to another computer, whether it is over a network or on hard media, the virus will immediately begin infecting the new computer as soon as the file or program is opened.
Boot Sector Viruses
Boot sector viruses are particularly volatile because not only can they infect a computer's hard drive like a regular file infector, but they can also automatically infect removable media such as floppy disks. The boot sector is the part of the hard drive or other disk where the information pertaining to the drive or the disk structure is stored. Boot sector viruses commonly render computers unable to boot properly (or at all).
Macro Viruses
By far the most common type of virus is the macro virus. The popularity of this type of virus is linked to the use of the macro programming language in popular products like Microsoft Office. Unlike the two aforementioned types of viruses, macro viruses do not infect programs at all, but rather they affect documents. Macro viruses are typically distributed via infected documents, and upon being open then infect the base template of the user's computer. As a result, each and every document opened from that point on will be infected.
Worms
In the virus world, worms are pieces of code that are capable of reproducing fully functional copies of themselves and then distributing themselves across a computer network or even across the internet. Instead of attaching itself to applications, like traditional viruses, a worm creates copies of itself, and those copies also replicate themselves, and so on and so forth. Worms are very powerful and unique in the sense that they can be created from simple code in a text editor and then become transformed with a different file extension. For instance, if you obtained a copy of a worm, you could change the file extension to ".txt" and then safely view the code that embodies the worm. Because of this ease of access, worms tend to be created and modified all the time, making them extremely prevalent online nuisances.
Trojan Horse Viruses
Borrowing their name from the wooden Trojan horse of Greek mythology that snuck soldiers into the city of Troy, a Trojan horse virus is any malicious program that surreptitiously invades a computer disguised as benign software. Sometimes Trojans are also found within other programs so that when the harmless program is installed, the Trojan is installed along with it. Some Trojan viruses can seriously compromise computers, giving full access to system files potentially to users in other parts of the world. The remote client is able to essentially seize control of the victim's computer, controlling it as if he/she were sitting right in front of it. Most Trojans are programmed to alert the owner of the malicious program whenever an infected user is online.
