In practice, though, there are three major components that play into the creation of viral attacks:.

: Hackers will attack where they have reason to do so. Consider the that attacked sensitive computers in Iran. If anyone knows for certain who wrote this, or how it was introduced into Iranian nuclear plants, they're not talking to . But these attacks weren’t motivated by the sudden increase of people walking around carrying programmable industrial logic controllers in their pockets.

: When there’s vulnerability in software—from Apple or or any other vendor—the malware hacker community is almost always the first to know. After all, this is their bread and butter—give them a chance to make money, and they’ll race to exploit the vulnerability before it’s found and fixed. Serious computer security experts (who are, alas, less frequently quoted in the general media than software vendors and supposed experts who give good sound bites) classify these opportunities into (theoretical avenues for attacks) and (actual attacks taking place in the real world).

For example, if you go away for the weekend and forget to lock your door, but you come home to an intact house, that's a vulnerability. But if you came back to find that some scoundrel has made off with your worldly possessions, thanks to your forgetfulness, that's an exploit. Most computer security issues you read about—and especially the ones you see on cable TV news—are vulnerabilities. But we're vulnerable to thousands of things a day, including death rays from outer space which do everything from to . Unless you've already lined both your hat and your laptop bag with tinfoil, you're clearly not too worried about this. (Nor should you be—the odds of this occurring are extremely low. And the tinfoil is unlikely to help.)

: is a concept from biological infection that also applies to computers. Disease relies on a certain critical mass to spread: if your population is dominated by those who are immune to infection, it helps curtail the communication of the disease. In terms of computers, it means that so long as the vast majority of machines that your computer interacts with aren't subject to the same malware—i.e. when your Mac talks to Windows PCs—you’re less likely to get infected. When your machine talks mostly to computers that  susceptible, herd immunity is lost.