You have to deal with them or they'll deal with you. Simply clicking through a website is all that's needed for them to keep hidden viruses into your Mac. More than that, adware and malware were programmed to be autonomous because they can coerce or cajole you into accessing websites where they lie in wait. They can give that same autonomy to your Mac because it can end up executing its own commands.
What does all this have to do with adware and malware? Well, that's just one of the effects they have on their targets. Technology and human ingenuity have given machines unprecedented autonomy because they end up executing commands of their own will. The machine you use today won't be the machine you use tomorrow.
