Computer Viruses
and other
Destructive Programs
Most people have never seen any damage caused by a computer virus before. Some people don't even believe in computer viruses. But they are a serious problem in our 'computer world'. The first document virus attack was in 1987 at the University of Delaware. The growth of the number of viruses is about 227% per year.
How can we protect our computer systems from computer viruses? To do this we must know a little more about these small programs.
A computer virus is a program that executes when an infected program is executed. It is able to clone itself - that means it can infect other programs with copies of itself. It is only a few lines of program code but they can do serious damage to other program files.
The difference of viruses and other destructive programs like Worms, Trojan Horses and Logic Bombs is that viruses can clone themselves. Even if other destructive programs can't do that, they can also do damage like a virus.
Harmless viruses only clone themselves and spread to new systems. Other ones can damage other programs or alter data. Some viruses are selfdestructing. They can delete themselves when they have done their bad work.
A virus can spread to a new system via interconnection. An interconnection is a connection of computers. This can be a network in a company, a connection with modems like the internet or just a floppy disk. Most viruses spread via infected floppy disks. Every contact between your system and any other system is an opportunity for infection!
There are five types of viruses:
Three out of every four virus infections are boot sector viruses. They infect the boot sector on the hard disk of the system. This makes it very difficult to detect the virus because the boot sector is the thing loaded when a computer starts. The only way to infect a system with a boot sector virus is to boot with an infected floppy disk.
File infecting viruses infect only executable files. These can be system files like the COMMAND.COM or programs. The infection usual increases the file's size. This makes detection a little easier.
This viruses consist of an algorithmic system. They change their appearance with each infection. Some polymorphic viruses can assume over two billion different guises. This makes it necessary for anti-virus software products to perform algorithmic scanning.
A stealth virus can hide changes it makes to file sizes. They can intercept all attempts to use the operating system because they stay and operate in memory.
A macro virus is a virus that is encoded as a macro embedded in a document. Many applications, such as Microsoft Word and Excel, support powerful macro languages. These applications allow embedding macros in documents. The macro executes each time the document is opened. 75% of all viruses today are macro viruses. Once a macro virus gets onto a system, it can embed itself in all future documents which were created with the application.
They are the worst viruses of all because they can combine some or all types of viruses. They can infect boot sectors and executable files.
The simple act of write-protecting floppy disks can prevent many virus infections. Use an updated version of anti-virus software to scan floppy disks before using. But an anti-virus software alone will not do the whole job. Education and training of users must be a part of virus protection.
A short instruction for protecting a computer system:
I. Any floppy disk should be write-protected before it is inserted in a disk drive.
II. Once the disk is write-protected, scan it for viruses before doing anything else.
III. Note any suspicious behaviour.
IV. Files should be downloaded from outside systems directly to floppy disks or to an isolated computer.
V. In a network dedicate a computer to virus control.
VI. Backup your data.
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