Do your research. We have been led to believe that vaccinations are mandatory for most foreign travel, but it is rather just recommended by many countries. The one that is most likely mandatory is for Yellow Fever, but usually that is only dependent on whether you are entering the country via a specific country which has Yellow Fever present.
When Layne was heading off to YWAM she was told that she was to have her vaccinations all up to-date. The staff was informed that she had forgone vaccinations since she was 3, and that she was not prepared to have them for attending their school, nor for travel (we already researched about the travel). We were told that she would likely have to sign some waiver, which she never did, and as far as she is aware she was the only student who wasn't vaccinated for travel. She traveled through Morocco, Spain, and Scotland, with stops in London and Amsterdam. There was never any problem with her not being vaccinated.
She had to stick to her guns as far as the school and the nurse, but she knew her grounds and rights. It was as simple as that.
As in other areas of your health, be educated about how your overall lifestyle affects your susceptibility to illness and disease, and take charge of your own health. Be diligent.