File infector virus: Most common form of the computer virus is the file infector virus, that hides its code within the code of the another programs. Its an executable program typically with exe, bat, pif, sys or com extension. By loading itself to the memory it runs separately and even can continue if the host program is closed. This type of virus is responsible for 85% of the infections.
Boot sector virus : It resides in the part of the hard disk which is read in the memory when the computer firsts boot up. since removable disks are less used today, boot sector viruses have become much less prevalent than they used to be.
Macro viruses : Come viruses are created with the macro coding languages. Macros are the small programs that are created to do highly specific tasks within an application. They are written in a pseudo-programming language designed to work with the application. VBA (example of macro language) is used in all microsoft applications. Thus they modify the file, send e-mails etc.
Trojan horse : A trojan horse is a program that claims to do one thing but does something very different. it looks innocuous enough to be safe to open but the moment we run the file it proceeds to inflict the damage on the system.
Worms : A worm is a program that scans the networks for another computer that has a specific security hole. It copies itself through that security hole and starts replicating. they don’t have to be delivered through the conventional programs therefore they are termed as fileless worms.
E-mail virus : A virus that is distribted theough e-mail. Many e-mail virus hijack e-mail program and send themself out to all the contacts in the address book.
Life cycle of the computer virus:
- Creation : Virus is created.
- Replication : Virus is copied from pc to pc.
- Activation : Virus launches and delivers destructive payloads.
- Discovery: Virus is detected and documented.
- Assimilation : Anti-viruses companies modify their programs to include new virus.
- Eradication : Use of anti-virus removes the threats.