If you are reading this, you are on a computer, you are on a network, and you fit somewhere in this continuum. You are also at serious risk. People make mistakes. People break into systems. Software has bugs.
Telephone lines go down.
These are a very few examples. They can originate from employees, the Internet and e-mail spammers. Viruses have even been found in software distributed by reputable publishers and hardware manufacturers. Some do it for profit. Others do it with the intent to cause malicious damage.