At Least Some Risk ... May Keep Us Safe

I've got Wireshark and I was thinking about whether I could use it to spot evil code (e.g. viruses, worms, etc.) using the WiFi connection. I could, but could some of these code-villians bypass this monitoring? Maybe most evil-code-doers already were.
It's almost certainly possible for them to bypass the tool that captures the network traffic (WinPcap), though it would involve manipulating the network devices outside of the operating system commands.
This is a stark example of the inevitable fallibility of any passive defense. But I realized that that fact is also a blessing, despite it being most remembered a curse. Walls are made to keep people in too. Freedom is abused by the vicious, but it's the only way to survive some evils we visit upon each other.
I've been reading Little Brother by Cory Doctorow about a near-future 'anti-terror' near-police-state San Fransisco and I'm appreciating the good that may be being done by people otherwise overloading our inboxes with spam.