In response to an outbreak of yellow fever in Uganda, the government of Tanzania (and Zanzibar) has stated that as of January 26th, 2011 a valid yellow fever vaccination certificate must be presented by ALL visitors at all points of entry into both Tanzania and Zanzibar. This changed from previous requirements which stated that yellow fever vaccination is required only for persons from or those who have visited yellow fever endemic countries.
INFORMATION FROM CDC.GOV:
Yellow fever can be prevented by vaccination. Travelers should also take precautions against mosquito bites when in areas with yellow fever transmission. Travelers should get vaccinated for yellow fever before visiting areas where yellow fever is found. Note: the vaccine is to be given 10 days before travel to an endemic area. If you continue to live or travel in yellow fever-endemic areas, you should receive a booster dose of yellow fever vaccine after 10 years.
In the United States, the vaccine is given only at designated yellow fever vaccination centers. International regulations require proof of yellow fever vaccination for travel to and from certain countries. People who get vaccinated should be given an International Certificate of Vaccination.