Yellow Fever is a mozzie borne illness found in tropical parts of only two continents, Africa and South America. The mortality rate is very high. The ‘yellow’ bit is the colour you turn as your liver packs it in. It is a miracle that the virus has never got into Asia, where the very same mozzie (Aedes) transmits up to 100 million cases of dengue per year. Imagine Yellow Fever getting into India or China, with a combined population of about 2.5 billion. The WHO has imagined that scenario, and is determined to avoid it by regulating for vaccination requirements at international borders. The enforcement of such regulations is done not directly by the WHO of course, but by each country. 

Yellow Fever vaccination is unique because it is the only vaccination that comes with a legal certificate which Australian customs may need you to show when you return from Africa or South America. That’s why you have to answer the question on your immigration card ‘have you been in Africa or South America in the last 6 days?’ The question alerts immigration to check your Yellow Fever certificate, if you've been in a risk country on those continents.

Below is the list of countries that Australia considers to be at risk for Yellow Fever, and from which returning travellers will need to show a certificate. Note every country in South America is listed except two, Chile and Uruguay. Note that it is only the tropical third of Africa which is at risk, not North Africa (Egypt, Morocco) and not Southern Africa (Botswana, Zim, Mozambique, Malawi are not in the zone.) Obviously it's a bit artificial and presumes mozzies respect political borders, e.g. Ethiopia is in the zone, while Eritrea and Djibouti are not.

Actually, the risk to an individual traveller in many of those listed countries is almost nil. Certainly there may be significant risk in those going to West African countries or to the Amazon, but there have been no cases in East Africa for years. Yellow Fever is the only vaccine where the decision to vaccinate is based not just on medical risk, but on administrative necessity. You do NOT want to find yourself at an African land border crossing without a certificate, being offered a Yellow Fever shot!

Under WHO regulations the certificate may be asked on your travels for as you arrive in a country. But each country is free to apply the regulations as they see fit, and as a result the rules between countries is complex and change from time to time. For instance India and South Africa seem to consider Zambia to be a Yellow Fever risk country while the WHO says it is not. The countries that are most likely to ask to see your certificate are those that have endemic Dengue Fever, but not Yellow Fever. This applies especially to most of Asia, Southern Africa and the Caribbean.

Yellow Fever vaccination is available only from WHO accredited clinics. You cannot get it on an ordinary prescription, as chemists are not permitted to stock it. A certificate will be issued, becoming valid 10 days after the shot, and valid for 10 years. There is scope for the doctor to provide an exemption certificate only if there is a genuine medical reason for not vaccinating, such as a traveller with a serious egg allergy, recent illness, old age*, thymus gland disorder (that's thymus, not thyroid!), cancer, chemotherapy, on steroids, or with an immune deficiency.

 * 'Old age' is medically defined by being older than the prescribing doctor! (OK. Concern about the vaccine's safety is higher after age 60 actually.)