Some countries require that you have paper evidence of certain medical tests, inoculations or prescription drugs before they allow you entry.
Below are some of the more common requirements for medical certificates:
Yellow fever - For some countries a valid yellow fever vaccination certificate is required for all travelers arriving from countries known to be infected with yellow fever.
Meningitis - Also, for some countries a valid certification of vaccination against meningitis is required for all visitors.
HIV Test - Some countries are also requesting that visitors take an HIV test prior to arriving in their country.
You might also consider taking a few sterile hypodermic needles with you in case they insist you need an injection for something when you arrive.
To find out which injections you need for where, or any other advice about inoculations, visit the section Inoculations. You can also contact the Consulate or Embassy of the country you are planning to visit for current requirements. Your local GP may also be able to find out which are needed for your destination.
If you are taking prescription drugs/medicines with you, it might help to get a note from your doctor on headed paper stating your need for the prescriptions he or she has given you.