Viruses

First, what is a virus? A virus is simply a computer program that is intentionally written to attach itself to other programs or disk boot sectors and replicate whenever those programs are executed or those infected disks are accessed. Viruses, as purely replicating entities, will not harm your system as long as they are coded properly. Any system damage resulting from a purely replicating virus happens because of bugs in the code that conflict with the system's configuration. In other words, a well-written virus that only contains code to infect programs will not damage your system. Your programs will contain the virus, but no other harm is done. The real damage--the erasing of files, the formatting of hard drives, the scrambling of partition tables, etc.--is caused by intentional destructive code contained within the virus. Generally, the destructive part of a virus is programmed to execute when certain conditions are met, usually a certain date, day, time, or number of infections. An example is the now infamous Michelangelo virus. This virus can run rampant on your computer for months and you won't notice that anything is wrong. That is because even though your hard disk's master boot record is infected with the virus, the destructive code has not yet been executed. The virus is programmed to trigger its destructive code on March 6, Michelangelo's birthday. Therefore, if Michelangelo contained no destructive code, nothing bad would happen to your computer even though it was infected with a virus.
An important thing to remember is that not all virus attacks produce catastrophic results. For example, one of the most common viruses in the world is called Form. I got Form from a floppy disk given to me by a friend who didn't know he had the virus. In fact, I didn't know I had it either until I received a call from a company to whom I mailed my resume using that floppy disk. They called me, not to tell me that I got the job, of course, but rather that my computer had the Form virus. How embarrassing! Apparently, Form had been on my computer for a long time, but its effects were so slight that I never noticed it. The only peculiarity I encountered was a clicking sound that emitted from my PC speaker every time I pressed a key, but this only happened for one day. Later, I learned that Form is programmed to trigger this action on the 18th of every month. Other than that, it doesn't contain any destructive code.
The only other time my system actually became infected was considerably more serious. It happened only a few months ago on the job. I was scanning a large stack of diskettes for viruses when I was distracted by a phone call. After completing the lengthy call I turned my computer off and took a short break. When I returned I booted my computer, forgetting that I had left a diskette in the A drive. I discovered my error when the floppy drive began to spin. At that point I also noticed that the disk was being accessed far too much for a non-system disk. Upon rebooting from the hard drive, I quickly realized my mistake. A virus called Junkie was all over my hard drive. It had infected command.com, as well as my screen reading software and all associated drivers. The Junkie virus was alive in the boot sector of the diskette that I inadvertently left in the drive, and it ran wild when I accidentally tried to boot from it. Junkie is a perfect example of a virus that, if written properly, would not have damaged my system. It contains no destructive code. It simply replicates by infecting .com files. However, not all .com files are structurally accurate. Without getting too technical, .com files are raw binary data read by your computer, and .exe files need to be interpreted first. There are some files, particularly ones used by memory management software, that have .com extensions, but that are actually written more like .exe files. When Junkie infects one of these types of files, it becomes corrupted because it is essentially an .exe file, but Junkie has appended .com-like instructions to it; similar to repairing a can opener with parts from a toaster.
After the near heart attack I had during my battle with the Junkie virus, I began to study the phenomenon very seriously, and since then, though I have run into many viruses on the job, none of them has infected my computer. This is because I now have an effective antivirus strategy in place.