It's easy to get hung up on the 'dangers' of some of these things but in reality you'll almost certainly have no problems. Don't let it put you off or worry you to death, but do look for some sensible advice.
When it comes down to malaria, the 'official' line will be
really cautious. If there was one suspected case in 1847, they'll have it down in the red zone. Looking at
this map it makes it look like Belize is a real hot spot. But I've never taken malaria medication down there and haven't known anyone else to either. It's definitely not a big issue in Belize.
Cancun is popular with American tourists and you can bet that none of them take malaria medication (or probably know that malaria exists). I imagine we'd hear about it if hordes of American tourists were coming down with malaria from that region. It's a personal choice but weigh up the 'risks' against the real possibility of fairly horrible side-effects from some anti-malaria drugs. I'd definitely take the drugs if it was a genuine malaria hot spot, but it isn't.
The best protection against malaria is not getting bitten. I imagine they have West Nile Virus down there as well (we've got it here in Tucson) so that's another reason not to get bitten by mosquitos. Use plenty of insect repellant and wear long trousers and long sleeves if you're going into the jungle.
Another thing to consider if you're off the beaten track is chiggers. These are annoying little bastards that start off like mosquito bites but instead of biting you the little feckers dig themselves into your skin and live there. You get them in particular by walking through long grass. They're not too dangerous but they are frustrating. I tend to get them once a summer here in Arizona. The odd trick with these is to cover the bites with nail varnish (an essential part of my jungle birding kit) which cuts off their air supply and kills them. Your body then gets rid of them no problem.
As for the other jabs, get them all and go back for the booster to get the ten year protection. With a bit of luck Matt, you'll be visiting more exotic locations over the next decade...