Mass travel and imported diseases.
D001359:Aviation; D002771:Cholera / Q000517:prevention & control; D003140:Communicable Disease Control; D006801:Humans; D010930:Plague / Q000517:prevention & control; D012899:Smallpox / Q000517:prevention & control; D014195:Travel; D015004:Yellow Fever / Q000517:prevention & control