You should always get shots before traveling. Everybody knows this, although seem to ignore the warnings. Of course, in an increasingly globalized world, many places you travel to are not going to have much in the way of exotic diseases. When I came to London, for example, I didn't worry about getting shots. Although if there was a shot that made you look the right way when crossing the street, I would've gladly paid for it. (I almost get run over ever time. I know, they even write which way to look at your feet. I still forget.)
Of course a lot of traveling, some might argue the best traveling, happens in places where the medical technology is not as good as it might be in much of the world. Shots are a smart idea in these places.
(The U.S. Center for Disease Control website has specific country vaccine recommendations. I know this because the last time I saw my Doctor and asked him what shots I should get for Colombia, he just sat down at his computer and read CDC recommendations to me from their website.)
But what people often forget is that although shots beforehand are a good preventative measure, there is always the chance they won't be enough, and if you’re in an area that is remote enough to need lots of vaccinations, then you probably won't be able to just stroll down to the pharmacy and get what you need. That's why it's not a bad idea to have a first aid kid with a good supply of drugs. (Please do not take this last sentence out of context.)
There's the basics of course, the over the counter stuff I always travel with. Pepto Bismol, Ibuprofen, Anti-Fungal stuff, and of course live leeches to even out the humors on the occasions I travel to the 18th century. But I'm sure there are many travelers who need specific non-over the counter medications, and if you are robbed or run out for any reason, you're going to have to get more. That's why I recommend carrying them in two different places, in case you are robbed, and also be ready to get more. I love this canadian pharmacy
for ordering stuff you might not be able to get where you're at.
The lesson, as always: thank god for the internet and Canada.