Computer viruses are unwanted software programs or piece of codes that interfere with the functioning of computer. They spread through contaminated files, data and insecure networks. Once it enters your system it can replicate to produce copies of itself. These copies of virus enable it to spreads from one program to other program of computer and from one infected computer to other computer.
Types of Computer Virus
Overwrite VirusOverwrite virus is the simplest computer virus. It overwrites the code of infected file with its own malicious code. The content of infected file is replaced partially or completely but the size of file does not change.
Macro virus alters or infects the macros of a document or data file. It is embedded as macro in a document and adds its codes to the macros of the document. The virus spreads when infected documents or data files are opened in other computers.
Boot virus or boot sector virus alters the boot sector program stored in hard disk or any other storage device. It replaces the boot sector program with its own malicious version. Usually it enters into your system through corrupt media files, infected storage devices and insecure computer networks.
Resident virus stays permanently in the primary memory (RAM) of computer. When you start the computer it becomes active and corrupts the files and programs running on the computer.
Multipartite virus spreads and infects in multiple ways. It infects both the files as well as the boot sector.
File Infector Virus
It is one of the commonly found computer viruses. It particularly infects the executable files; the files with .com or .exe extensions. The virus becomes active when the infected file is executed. The active virus overwrites the file partially or completely. Thus it may destroy the original file partially or completely.
Computer worm is similar to virus but is technically different from virus. It can replicate and spread like virus but unlike viruses it don't need a host program to spread. Being able to self replicate it can produce multiple copies of itself. It spreads through networks such as an email sent to an infected email id can infect your system with computer worm.
Trojan horse is a malware like a virus or worm but it is technically different from both. It can't replicate like virus and worm. Trojan horse hides itself in a program. Once you install any such program the trojan horse enters into your computer. It can provide unauthorized access to your computer, send your files to other computers and may delete files or can make other unwanted changes in your computer.