A Virus is a program which reproduces itself, hides in other computer code without permission and does nasty or undesirable things, not intended by its victim.
A Virus is a parasitic program that infects another legitimate program, which is sometimes called as Host.
The virus modifies the host so that it contains a copy of the virus.
Some types of viruses are relatively harmless. Such viruses are described as benign.
Other viruses are indeed malicious, and can do great damage to a computer system if permitted to run.
Trivial, simply reproduces or displays messages.
Minor, alters or deletes infected files.
Moderate, wipes out entire disk drive.
Major, slowly corrupts data with pattern, making restoration difficult.
Severe, slowly corrupts data without pattern, making restoration impossible.
Unlimited, virus which discovers system administrator’s password and mails it to one or more users, tempting them to use it for illegal purposes.
Types of Viruses
1 Boot Virus
Infect the boot block on a floppy or hard disk.
Usually replaces the boot block with all or part of a virus program.
Most have trigger dates, when booted on that day severe damage will be done.
Virus loads into memory and infects other disks.
The virus allows the actual boot sector data to be read as though a normal start-up were occurring.
Example: Michelangelo – On March 6 (Michelangelo’s birthday), garbage is written through entire drive.
2 Cluster Viruses
Make changes to a disk’s file system. If any program is run from the infected disk, the program causes the virus also to run. This technique creates the illusion that the virus has infected every program on the disk.
A worm is a program whose purpose is to duplicate itself. An effective worm will fill entire disks with copies of itself, and it can spread to multiple computers on a network, essentially clogging the entire system with copies. Worms are commonly spread over the internet via e-mail attachments and Internet Relay Chat (IRC).
4 Trojan Horses
A Trojan Horse is a malicious program that appears to be friendly. For example, some Trojan Horses appear to be games. They do not make duplicates of themselves on the victim’s disk or copy themselves on other disks. But since they can do harm, they are considered viruses.
Whereas Viruses reproduce, a Trojan is just a one time program which executes its payload as soon as the Trojan is executed.
Example: pkz300b.exe, which disguises itself as an archiving utility, but when run it will delete your entire hard disk.
5 Macro Viruses
A macro virus is designed to infect a specific type of document file, such as MS Word or Excel files. These types of documents can include macros, which are small programs that execute commands. Such documents are usually transmitted through e-mail attachments and multiply themselves through the address book of the recipient’s mail account. It can do various levels of damage to data, from corrupting documents to deleting data.
6 Joke Programs
Joke programs are not viruses and do not inflict any damage.
Their purpose is to frighten their victims into thinking that a virus has infected and damaged their system.
For example, a Joke program may display a message that says the computer’s hard disk is being reformatted.
- Never use a ‘foreign’ disk or CD-ROM without scanning it for viruses.
- Always scan files downloaded from the internet or bulletin boards.
- Never boot your PC from a floppy unless you are certain it is virus free.
- Write protect your disks to prevent viruses from reproducing onto your disks.
- Use licensed software from a reputable dealer.
- Password protect your PC to prevent copying of files in your absence.
- Make regular backup copies of all your work and system configurations.
- Install and use anti-virus software regularly.
- Update your anti-virus software regularly so it can detect new viruses.