Travel Vaccination Certificates
Some countries require you to have an International Certificate of Vaccination or Prophylaxis (ICVP) before you enter.
For example, Saudi Arabia requires proof of vaccination against certain types of meningitis for visitors arriving for Hajj Vaccination and Umrah Vaccination, Pakistan requires proof of Polio vaccination if you are planning to stay for over 4 weeks, and many tropical countries in Africa and South America will not accept travellers unless they can prove that they have received the Yellow Fever vaccine.