a) Computer viruses has ability to replicate them self into large number of computer. Spread by floppy disk and USB drive or nowadays is through internet.
b) Malicious programs may be installed by hand on a single machine. But may built into software packages. It likes Trojan Horse, Traps Doors and Logic Bombs.
Malicious program split into two:
* Need host program - Trap doors, Logic Bombs, Trojan Horses, Viruses.
*Independent - Bacteria, Worms.
:: Examples of malicious code
1) Trojan Horse -
2) Virus - code segment that replicates and attach to existing programs.
Resident - stay in program when execute, stay till it trigger again.
3) Worm - program that replicates and copy the file/folder it locates.
4) Bacteria - program that replicates itself until fills up all spaces or CPU cycles.
5) Logic Bomb/ Time Bomb - malicious code that activates on an event.
6) Trap Door - written entry point, that cause unwanted users.
7) Spyware - Steal info, see web sites visited, see contents of files, popUp ads, slow down computer and crash the computer too.