You should get vaccinated against any diseases that may be endemic to the region where you're headed at least four weeks before a trip out of the country. That's how long it takes most vaccines to provide your body with full immunity against germs. Even if you're rushing out of town on a last minute business trip without a lot of time to prepare, it's a good idea to get vaccinated against local diseases before you go, even if it means a quick rush to the doctor's office when you'd rather be shopping for a nice tie to wear to that important meeting. The last-second vaccination may lessen the severity of any illnesses you contract.
The Centers for Disease Control and Prevention (CDC) Web site has important information about what diseases you should be vaccinated against before traveling to certain parts of the world. They divide the vaccines into three classes: routine vaccinations, recommended vaccinations and required vaccinations. That last group is the most important because you're literally not allowed to leave the country without them. For instance, at the time this article was written, yellow fever vaccinations are required for travel to sub-Saharan Africa and parts of South America.
Of course, if your family plans to travel abroad at some indeterminate time in the near future it may be best to go ahead and get the recommended vaccines now, to avoid the last minute rush.