More about computer Virus

Virus Damage
In most cases, viruses can send our data to a third party and then delete our data from our computer. They can also ruin our system and render it unusable without a re-installation of the operating system. Usually the virus will install files on our system then will change our system so the virus is run every time you start our system. It will then attempt to replicate itself by sending itself to other potential victims.
The normal effect a virus will have on our system is that over time our system will run slower. Also when you are using the internet our connection may seem to run slower. Eventually you may have trouble running programs on our system, our system may freeze, and in the worst case you may not be able to get it to boot up when you turn our computer on. Some examples of virus damage are as follows;
• Releasing the Melissa virus, that he caused over 80 million dollars in damage.
• The “I Love You” virus was propagated from the Philippines in May 2000.20 Estimates of the damage it caused range up to $10 billion, mostly in lost work time.

Virus Behavior
Viruses come in a great many different forms, but they all potentially have two phases to their execution, the infection phase and the attack phase:
Infection Phase
When the virus executes it has the potential to infect other programs. Some viruses infect other programs each time they are executed; other viruses infect only upon a certain trigger. Many viruses go resident in the memory of our PC in the same or similar way as terminate and stay resident (TSR) programs.
Attack Phase
Just as the infection phase can be triggered by some event, the attack phase also has its own trigger. This means the attack could be delayed for days, weeks, months, or even years after the initial infection.
Types of Viruses
Computer viruses come in a variety of types. Breaking them into categories is not easy as many viruses have multiple characteristics and so would fall into multiple categories. We're going to describe two different types of category systems: what they infect and how they infect. Because they are so common, we're also going to include a category specific to worms.
System Sector Viruses
These infect control information on the disk itself.
File Viruses
These infect program (COM and EXE) files.
Macro Viruses
These infect files you might think of as data files. But, because they contain macro
programs they can be infected.
Companion Viruses
A special type that adds files that run first to our disk.
Cluster Viruses
A special type that infects through the disk directory.
Batch File Viruses
These use text batch files to infect.
Source Code Viruses
These add code to actual program source code.