Every computer can be infected with such materials - not necessarily intentionally. Here is an example of how it can be trivially done with no tools whatsoever.
Here, between these brackets, is an image: [
] - can you see it? Its visible width and height is set to 1x1 pixel. Your eyes probably will ignore a speck of dust like that. But your computer downloads the entire image and stores it in the cache in its original format, however large it may be. (In this case it's 800x587 pixels.) This image is technically "on your computer," and there is no difference whatsoever if you downloaded it to watch in full size, or you downloaded it because it's part of some Web page and you never saw it. It's not trivial to prove that you haven't browsed to that image intentionally. The proof would require showing the Web page that contained the <img src="..."> tag with that URL. You are mostly OK if you are an expert in computers and know what to look for; or if your lawyer can hire a competent specialist. Everyone else will be convicted because the prosecution will not look for exculpatory evidence, and the public defender has no time or money to spend on digging the facts up.
I used this innocent image from Wikipedia for this example. But the attacker can use any image that he likes - even images that are not publicly known or seen by anyone. To plant this mine he only needs to write a HTML message, just like this one, and wait for the victims to collect the illegal payload, all without knowing it and without even being able to know about it.
There are countermeasures. The easiest, and the only one that a non-geek can afford, is to configure the browser so that all temporary files are removed after you close the browser. This does not entirely eliminate the cache because deleted files can linger for months on a large drive. But going into secure wipe software gets complicated fast. Seasoned geeks would browse in a VM with reverting to a snapshot after they are done. Another way to do it is by using a Live CD distribution of Linux, where nothing that you browse is saved on the hard disk. Firefox has some add-ons that can control automatic loading of images from some sites but not from other. Blocking ads entirely (using AdBlock and Ghostery and NoScript) would be a good first step because ad networks do not screen submitted ads, and from time to time undesirable images are sent to millions of customers.