Where are all those people on planes going? Do they just stay in the airport forever? No! They go in to town and have meetings and play at the amusements parks and visit with their friends and family and eat at restaurants. I am sure you and your child has come in contact with someone from another country at some point in time, or will. If it was really a concern I think we would be seeing a lot more of these diseases.
It also brings up the idea that foreigners are dirty. The idea drives me batty! They are not carrying all sorts of diseases, and if they are I am willing to bet its not something we vax for (a disease for which you are a carrier but wouldn't necessarily know it like typhoid).
Further more if she believes vaxes work than she should have no fear of someone coming from another country bringing with them a case of a VPD - cause lots of countries have higher rates of vaccination than America.
And still how many of these people are actually traveling sick? Like with WC or Measles, or mumps or cp? Really? I think if you showed up covered in spots they might not let you on the plane.
And if you really want to put her mind at ease, in the last 30 years there have only been 8 cases of imported Polio (compare that to the number of cases actually caused by the vaccine).
But I personally would just tell her that I hope her vaccines are up to date cause you know adults can get diseases too. Hmmm like typhoid.