1. VIRUS

2. DEFINITION
A computer virus is similar to the biological viruses that cause disease on a daily basis.
Just as a biological virus is a small strand of packaged DNA that’s sole purpose is to invade a host body and use it to replicate itself,
so too a computer virus invades a host computer and then replicates itself.
Often viruses do damage, slowing down a computer, erasing files, and even damaging computer hardware.

3. WORKING A computer virus performs two basic functions:
it copies itself from machine to machine (self- reproducing), and
it executes the instructions the perpetrator has planned.
A computer virus exhibits three characteristics:
a replication mechanism,
an activation mechanism, and
an objective.
The replication mechanism searches for other programs to infect, if it finds any, it inserts the hidden instructions in the program then it modifies the execution sequence of the program's instructions such that the hidden code will be executed whenever the program is invoked.
The activation mechanism checks for the occurrence of some event. When the event occurs, the computer virus executes its objective.

4. DAMAGE
A computer virus is only one of the many kinds of threats to the security and integrity of computing resources.
Like other threats, a computer virus can cause unwanted alteration or loss of data.
The damage done depends on the computer virus.
There are viruses that do no harm, others
That just annoy the user, and
still others that can completely destroy all your data files.
In some cases, your machine can even be rendered unusable.

5. SPREAD
Viruses are usually spread by media/programs swapped through electronic bulletin boards or the internet.

6. SYMPTOMS
Some computer viruses alert you to their presence by putting an unexpected message on your screen.
Others use more subtle methods.
Things to watch for:
Programs taking longer to start or running more slowly than usual
Disk accesses seem excessive for simple tasks
Access lights turn on for non referenced devices
Unexplained decreases in the amount of available memory or system resources
Programs or files mysteriously disappear
A sudden reduction in disk space
Executable files change size and unexplained hidden files appear

7. Types of Virus Time Bomb Logical Bomb Worm Boot Sector Virus
Macros Virus
Script Virus
Trojan Horse

8. Time Bomb
A time bomb is a virus program that performs an activity on a particular date.

9. Logical Bomb
A logical bomb is a destructive program that performs an activity when a certain action has occurred.

10. Worm Virus
A worm is also a destructive program that fills a computer system with self-replicating information, clogging the system so that its operations are slowed down or stopped.

11. Boot Sector Virus
A boot sector virus infects boot sector of computers. During system boot, boot sector virus is loaded into main memory and destroys data stored in hard disk.

12. Macro Virus
A macro virus is associated with application software like word and excel. When opening the infected document, macro virus is loaded into main memory and destroys the data stored in hard disk.

13. Script Virus
Commonly found script viruses are written using the Visual Basic Scripting edition (VBS) and the JavaScript programming languages.

14. Trojan Horse
Trojan Horse is a destructive program. It usually pretends as computer games or application software. If executed, computer system will be damaged.

15. Trojan Horse
Trojan Horse usually comes with monitoring tools and key loggers

16. Actions to prevent virus infection
Always update your anti-virus software at least weekly. Back up your important files and ensure that they can be restored. Change the computer's boot sequence to always start the PC from its hard drive

17. Actions to prevent virus infection
Don't share Drive C: without a password and without read-only restrictions. Empty floppy drives of diskettes before turning on computers, especially laptops.

18. Actions to prevent virus infection
Forget opening unexpected attachments, even if they're from friends. Get trained on your computer's anti-virus software and use it. Have multiple backups of important files. This lowers the chance that all are infected.

19. Actions to prevent virus infection
Install security updates for your operating system and programs as soon as possible. Jump at the chance to learn more about your computer. This will help you spot viruses.