Chapter 5 Question 2

2. How do viruses spread over the Internet? How do you stop them?

Viruses are code that must be executed. They can only spread if people run the code either as a macro language/script or as an executable file. The only way to truly stop them is to never run code that might contain a virus. Modern systems tend to block common types of code (macros and scripts) and not allow people to open them. Code signing is one option to allow acceptable code.
You must also keep your operating system and software up to date with patches. If you leave a system open, it can be attacked even if you do nothing more than leave it connected to the Internet or browse certain Web sites.