There is a very good book, Traveler's Health, by Dr. Richard Dawood that you may want to read if you are going to be doing a lot of foreign travel. In it he has tables of all diseases and areas of the world affected. Also has good tables on which vaccinations/immunizations for a given area. I don't generally like to read those kind of books but found this one fascinating, maybe because I go to so many places and it's nice to be aware of what's out there.
In addition to what G200 said, and depending on where you are going, you need to look at Meningitis and Encephalitis. At a minimum I wouldn't travel overseas without a Tetanus/Diptheria booster and a Hepatitis A shot. The current Havrix vaccine for Hep A is a two shot series but you have good protection after the first shot. I would say it's a good idea to get MMR and Polio boosters as well. If you are going to be swapping bodily fluids with the indigineous population then a Hepatitis B series would be in order, especially for travel to Eastern Europe and Asia.
Malaria is another one to be aware of if you are travelling to Africa or parts of Asia. I won't ever take Lariam ( mefloquine ) again, it made me wicked dizzy. If I have to I'd try Fansidar, but try finding a doctor in the states that even knows what that is. Another choice would be a Doxycycline tablet daily. That and a gin and tonic every evening and you'll be okay
You are looking at about $75.00 per shot at a private medical clinic, if you can find a county medical facility or other public facility that offers these shots they can be much cheaper. We used to have people in our crew room giving the Hep A and B shots for about $10 to $20 each.
An addendum on TB. The vaccine isn't usually given in the States. The BCG vaccination is still pushed on me by English doctors but I'd really rather not test positive for it the rest of my life. That and risk a fairly big scar on your arm. The efficacy of the vaccine is not very good and you are still at risk of contracting the disease. For all those reasons I refuse the vaccine when offered. Just try to stay out of crowds of poor people hacking their lungs up.
Typhoonpilot