What's up with viruses and worms?

I know I'm old and stupid, but what's up with viruses, worms and trogan horses? Who puts them in there and why do they do it? Are these people who don't have a life, or is it something I'll never understand?

What's up with viruses and worms?norton 360

VIRUSES:

A virus is a malicious computer program or programming code that replicates by infecting files, installed software or removable media. A virus usually carries a destructive payload, which varies depending on the virus author鈥檚 intentions. A typical virus infects, corrupts or deletes files and folders, damages the system, drops other dangerous parasites, steals or discloses user sensitive information. Extremely dangerous viruses can also wipe out all the data from hard disks and even severely damage certain computer hardware devices.

By replication approaches viruses are divided into three main categories:

1. parasites called file infectors are designed to propagate by infecting or corrupting various files;

2. threats known as boot record infectors spread through removable media containing infected executable code and insert themselves into the master boot record (MBR) on hard disks;

3. widely spread macro viruses affect certain applications such as Microsoft Word or Microsoft Excel and infect documents that can contain macros.