Yellow Fever Vaccination
Because yellow fever is endemic in many tropical areas
of South America & the central band of Africa (but not Asia), most countries in those
regions require proof of yellow fever vaccination from all arriving
travellers - yellow fever is unique is this regard. Without proof of
vaccination, i.e. a certificate, you run the risk of being refused entry into a country.
Yellow Fever is a viral infection transmitted to humans by the bite
of infected mosquitoes. Yellow Fever gets it's name from the yellowing of the skin
and/or eyes which occurs in the later stages of the disease.
Yellow Fever (YF) is said to be endemic in many developing
countries, this simply means that YF is present to a greater or
lesser degree in the country's population, i.e. the disease is
ongoing in that locality... having said that, many yellow fever
infections are mild, causing fever, headache, muscle pain and vomiting, but the infection can
become more serious, causing bleeding and other life-threatening heart,
liver and kidney problems. Symptoms begin 3 to 6 days after infection.
Yellow fever is unique in that getting vaccinated and being able to
prove it is mandatory for the majority of African and South
American countries, I'm not entirely sure why YF is different from say,
Tuberculosis or Typhoid
in this regard (which are recommended but not mandatory
vaccinations) - but you should certainly get vaccinated for YF if you are headed
anywhere near an infected country - your doctor's surgery or travel
clinic will tell you if you need to have the jab and will issue the
certificate. Keep the Yellow fever vaccination certificate with your
travel documents, it's as important as your
passport in affected countries.

Back in the day, a yellow fever certificate really was just that;
an A5 size, single sheet yellow/green certificate, nowadays it's a
general purpose (albeit yellow) booklet written in English and French
that could conceivably be used to certify any vaccination.
The YF vaccine probably won't be available for free in your country
(it's certainly not available on the NHS in the UK) and it's one of the
more pricey injections available from your Travel Clinic at ~£50/USD~$75.
On the plus side, the vaccine is effective for at least 10 years.
Any guidebook you care to mention will tell you how important your YF Certificate
is, I won't argue with that; however
in my experience it's rare to be asked to show the Certificate,
even on a big multi-country trip. Don't be complacent though - I was
embarrassingly caught out in 2011 when entering Mali overland; the
border guards (who had their football game interrupted by the arrival of
our bus) requested YF Certificates for all passengers, everyone had them
to hand except yours truly, who spent 10 frantic minutes scrambling up
to the roof of the bus, pulling my kit apart in search of the
certificate.
In addition to getting the YF vaccine, you can help protect yourself
against yellow fever by preventing mosquito bites (see the
Avoiding Mosquitoes page).