computer viruses are not structured to exist by themselves.
A virus code executes when the programs to which they are attached or executed
Malicious viruses may delete a files or corrupt or cause sysytem to became unstable.
Some viruses are just spread themselves to other systems without performing any malicious outs.
Types of Viruses
1.Memory resident virus
Lodges in main memory as part of a resident system program.
2.Boot sector virus
Infects a master boot record and spreads when a system is booted from the disk containing the virus.
A virus that mutates with every infection making detection by tne signature of the virus impossible.