What is a virus?
A VIRUS is a small, executable program with the ability to replicate itself by adding its code to that of a host program or the system area of a hard disk. The user is generally unaware of the actions of a virus as it replicates and usually only becomes aware of its presence when the virus 'activates', which it does according to a given set of conditions.Once the user knows what signs to look for, it can be very obvious when viral activity occurs... Let's discuss the difference between viruses.....
Every virus has its own personality. Viruses differ in many ways, each having its own unique properties that make it different. Here are some ways that viruses differ from each other:
- SIZE - A virus can be as small as 66 bytes or less, or as large as 4096 bytes or more. Compared to most computer programs a virus must be very small.
- METHOD OF INFECTION - A virus can infect the host program in different ways. Below are three methods commonly used. They are by no means the only ways, but they are the most common. It is possible for a virus to use one or more of these methods.
- OVERWRITING - When a virus infects using this method, it will simply write a copy of itself over the begining of the host program. This is a very simple method and is used by more primitive viruses. Overwriting tends to make the user suspicious because the host program no longer functions. This method of infection causes no change in the size of an infected program.
- APPENDING - This method is a bit more complex. The virus appends itself onto the end of the host program and also edits the begining of the program. When the user runs the infected program ....it will jump to the end of the program where the virus is located, perform the functions of the virus, then return and continue to run the host program. To the user, the program is functioning normally. This method of infection causes infected programs to increase in size. Some appending viruses are unable to tell whether or not they have already infected a program and will continue to infect the program hundreds of times, causing it to grow considerably in size.
- DISK INFECTORS - Other viruses will infect the boot record or partition table. This is an executable area of the disk that is automatically run every time you boot up from the disk. This means that as soon as the computer boots up, the virus is in memory.
- TSR -(transient but stay resident!) A virus may or may not become resident in memory. If it does go TSR, then its chances of infecting files are greatly increased. Otherwise it can only do its stuff when an infected program is run. If the virus is in memory it can infect files any time it chooses. Partition table and boot sector infecting viruses are always TSRs.
- STEALTH - Some TSR viruses use a sophisticated technique called Stealth cloaking. What this means is the virus will fool the system so that everything appears to be normal. When a user does a directory listing the virus will intercept the disk read, and alter the data so that the file sizes appear to be unchanged, when in actuality they have increased in size. Boot sector infectors may use stealth so that when the user attempts to view the boot record, instead of showing the actual boot record, a copy of the old boot record is returned instead. Because of stealth techniques it may be impossible to detect a virus once it has become resident in memory.
- ACTIVATION CRITERIA AND EFFECT- The other area that gives a virus its personality is the activation. Some activate by the date, others activate when a certain program is run, and other will activate when they can't find any more files that haven't been infected yet. When a virus activates it will take a certain action.
credits: google
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