To attach might mean physically adding to the end of a file, inserting into the middle of a file, or simply placing a pointer to a different location on the disk somewhere where the virus can find it. This self-replicating code, when triggered by some event, may do a potentially harmful act to your computer. Another way of looking at viruses is to consider them to be programs written to create copies of themselves. Similar to viruses, you can also find malicious code in Trojan Horses , worms , and logic bombs . Number of Viruses Lots and lots.
Worms spreading due to user inattention are a serious threat.
The general consensus is that there are none. Hardware Threats Viruses are not the only things that can cause damage. Consider some hardware problems. Software Threats Viruses are not the only things that can cause damage. Consider some software problems.