Knowing your enemy is half of the fight
As the internet changes, both in terms of the technology that underpins it and the way in which we use it, so too does the type of viral threat we can expect to encounter. The 'mischief' viruses of the past gave way to commercially motivated adware and spyware, and so too have these been succeeded by ransomware and scareware - programs that rely on social engineering to survive instead of semi-legitimate business.
The sheer number of threats out there means that making sense of them all can be confusing. What's the difference between a virus and worm? What does adware do that scareware doesn't? And how do you even end up infected with these different types of malware?