It is a computer program that has written intentionally and can attach itself to the disk boot sectors or any other programs. The virus is placed in the programs that are most widely and commonly used so that it can spread very fast. Whenever you run that particular program, virus also runs and can attach to the other programs too. Viruses are replicating entities and cannot cause any harm to your computer as long as they are coded in a proper way. Your system gets any kind of damage from replicating virus because of bugs in code that creates conflict with system configuration. A virus cannot affect your computer until the infected program is executed or you use a disk that contains infected files. When the virus becomes active it gets into the memory of computer or into the system files or applications.
Some viruses only display messages, create sound or comments at different time intervals after replicating themselves or whenever you run the affected program. They cannot do more than this while other viruses are made in such a way that they can corrupt program, delete files or can delete all the data on the hard disk. Some viruses can crash or compel your computer to behave erratically. Generally people cannot recognize that it is due to the virus in the computer and remains with it until the virus is detected.
There are different types of viruses:-
Macro is a code that can remains in the data file, and macro virus is a virus that remains in the data file (i.e. document) as its macro attachment. The macro viruses can attach only to the data files and not to the executable programs. These viruses were very common in the end of last century.
When this virus is active it makes some changes in the file and hides those changes. So the programs that read the files only see the original form and not the infected form. That is why antivirus cannot detect it easily. Virus should be present in the memory to trace it through antivirus.
This virus can create different operational copies from itself. Simple scanners are not able to find out all the operational copies of the virus.
BOOT SECTOR VIRUS
This virus infects the master boot records (MBR) or DOS boot sector. Master boot record is always on the first sector of the hard disk. When the virus goes into the drive it gets loaded in the memory every time we start the computer. It further goes to every disk of the system. It is not easy to get rid of boot sector virus because most of the antiviruses are not able to remove this virus or clean up the master boot record during the working of windows. Bootable antivirus disk is required to properly clean the MBR and get rid of boot sector virus.
Worms are the programs that have the ability to replicate themselves but they cannot attach to a file or program and always remain as a separate thing. Many times they have functionality that can disturb the normal working of a program file. They are very much similar to viruses and are taken as the virus instead of worm.
Trojan horse cannot replicate itself automatically. It is a simple program for e.g. game but in actual it causes harm to your computer. It can even erase data from the computerâ€™s hard drive. They can be detected easily.
This virus can automatically replicate itself and moves on by attaching itself to an email. This virus mails itself to the people in the infected peopleâ€™s address book. Some of these viruses do not even need a double click. They simply get loaded when you see the preview pane of your mail software.
This virus gets active on a particular date and time and they can cause harm by deleting files.
These are the files that have commands which load the virus into the computer. They cannot be easily detected.
DIRECT ACTION VIRUS
They do not reside in the memory but only cause destructive harm.
Virus cannot infect compressed files, computer hardware like- keyboard, monitor, mouse etc. They are only harmful for the software.