Essentially what happens is that the use of normal antibiotics kills off the good bacteria in your gut. This allows the bad bacteria (c.diff) to overgrow, as the PP said. Emily gets really sick with c.diff...letharagic, criees alot, in pain...it's not fun.
We have found that taking a daily pro-biotic goes along way in keeping c.diff away. A pro-biotic is basically a dose of live active cultures...the good bacteria that is in all of our stomaches. Emily takes Florajen daily and her c.diff flares much less frequently, ,now.
Also, since she can't just "stop" anti-biotics usually, we are now in the habit of having a scrip for the c.diff med (usually Vancomyacin for us) called in whenever she goes on an oral anti. We don't always fill the scrip, only if she seems to be symptomatic.
There are some people for whom the c.diff never really goes away, they just have more or less severe symptoms at any given time. It's a tought one!