Update April 1, 2016: All visitors travelling to Rwanda are now required to take the Yellow Fever Vaccination. The Yellow Fever Vaccination certificate is required for all travellers arriving from all countries. This update is due to an increase in yellow fever cases in the East Africa.
If you arrive in Rwanda (specifically Kigali International Airport) without a valid yellow fever certificate, you will have to pay US $40 to receive the vaccination on arrival.
The Yellow Fever vaccination certificate can be obtained from your country of departure (usually a government institute) for a nominal sum. Upon receiving the vaccination you will receive a certificate issued by WHO which is valid for 10 years.
To learn more about the Yellow Fever vaccination certificate click here.