I've spent the last few days in self-imposed isolation to avoid spreading this filthy cold. You could call it quarantine - but you'd be wrong! I haven't been locked away for nearly long enough. (Oh shut up, you at the back, you know what I mean!)
Back in the Middle Ages, when men wore tights and silly hats, and talked like Errol Flynn, plague was a regular visitor. At the first sign of plague, sensible city aldermen would lock the city gates, and not open them until a specified period of time had passed. Of course, there would be pressure from the local merchants to open the gates as soon as possible so trade could resume. (Yep, it's that same old corny plot device you get in Jaws. Plus ça change and all that.)
There were two theories, one that you should shut your gates for 30 days, known as trentine, and one that you should shut your days for a far more biblical 40 days and 40 nights, known as quarantine. (Remember your schoolboy French or Latin? It's obvious when you think about it.)
Guess which one works, and which one ends up with half your citizens dead? And that's why these days we quarantine infectious people, we don't trentine them.