A computer virus is a program that spreads across computers by attaching a copy of itself to the files on your computer. When you run the infected file the virus gets into action. A virus is usually harmful and can corrupt data, overwrie files, or use up system resources, and slow it down in the process.
Where do viruses come from?
Viruses are usually written by programmers to cause trouble.
Types of Viruses:-
There are many types of computer viruses:
a) viruses which attack certain area or location and
b) viruses which attack in a particular manner
(a) The major types identified depending on the area they infect are:
i) Boot Sector Virus:-
These viruses infect the boot sector of the disks. A boot sector stores the program that have to be executed when you start your PC. By infecting this area of the disk, the virus loads itself when you boo the disk. It then remains in your PC's memory and infects any other disk you use on your system. There is more dangerous virus type, you have to watch out, the Partition Sector Virus. It stays in an area called Master Boot Record (MBR) which stores the information about the partitions on your hard drive.The partition sector also has a program that is run every time you power up your PC. So when you start your PC the virus is loaded in to memory straight away. Some viruses also encrypts the contents of the Master Boot Record i.e. the contents of your hard disk cannot be accessed any more.
ii) File Viruses:-
File viruses attach a copy of themselves onto the beginning of an executable file(files with .COM or .EXE extension). When you run the infected file, the virus code gets executed. The more intillegent viruses then execute the actual file-so everything appears normal. But the virus is now in our PC's memory. It soon infects most of the programs on your hard disk. So all your precious data is at great risk.
iii) Cluster Viruses:-
Cluster viruses store a single virus copy in a certain area on the disk. Then they change the DOS directory information so that thee location of each program in the File Allocation Table(FAT) points to the area where the virus sits. When you run a program, the virus gets to work and then it runs the original program. And this increases the chances of your data getting corrupted.
(b) The main ones that attack the computers in a particular manner:
i) Stealth Viruses:
These viruses try to cover their presence by hiding the modifications they make. When any application asks for some disk information-file sizes, free disk space, and the virus interrupts it and gives back the orginal information instead of the actual information.
ii) Polymorphic viruses:-
Most virus scanners look for "search strings" within a program to detect viruses. Polymorphic viruses encrypt their code on each infection so that the virus codes of two infected files don't look the same. So detecting these viruses is much harder.
iii) Multipartite virus:-
These are particularly nasty. They can infect both boot sector as well as files. So they spread more quickly than other viruses. Forunately, their occurence is rare.
iv) Fast, slow, and sparse virus infecting viruses:-
A fast infecting virus infects files whenever they are accessed. So whenever you read file, or write or modify it, or judt do a directory listing, the file gets infected. This makes them easier to detect but they can be dangerous too. When you run a virus scan all the files are read. If your virus scanner is out dated and can't cath the virus, your entire hard disk will be infected. Slow infecting viruses infect files only when you modify or create files. Sparse infectors get into action only under certain conditions. For example, such a virus will infect a file only on certain date.
