These are areas where the virus is present in monkeys and is a potential risk to humans as defined by the World Health Organisation. Some of these countries demand a yellow fever certificate from travellers as a condition of entry to their country. Many of these, and other countries, will ask you for a certificate if you are entering from an infected country.
You are advised to visit your general practice surgery or a travel medicine clinic at least 6 weeks before you travel. However, it is never too late to seek advice.
If you have a medical condition, you are advised to discuss the suitability of the trip before you book.