Hear  the audio explanation from the CDC

Yellow Fever
Yellow fever is a flavivirus that can cause severe symptoms in travelers

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There are 200,000 cases of yellow fever per year with 30,000 deaths. There are 3 modes of transmission of this virus by mosquito: sylvatic, intermediate and urban. The sylvatic cycle occurs in tropical rain forests. The virus is found in monkeys who mosquitoes bite and spread to people. The intermediate cycle occurs in the humid / semi humid savannahs of Africa. Both humans and monkeys are reservoirs for yellow fever. The urban cycle occurs when immigrants infected will introduce the virus into a population where mosquitoes that were formerly virus free pick it up.

Many countries require a  yellow- fever certification of  vaccination from arriving travelers to protects that country from travellers who may be bringining in yellow fever. Resident Aedes egypti mosquitoes may become infected with the virus and cause an urban yellow fever outbreak.

In other countries or regions within countries yellow fever may be recommended for travelers based on an assessment of their actual risk from their itinerary

Symptoms of yellow fever, range from none, to full blown cases. The incubation period after being bitten is 3-6 days followed by fever, headache, muscle aches, and protein in urine. Usually a slow pulse with fever is noted and abdominal tenderness. After 3-5 days people get better or may deteriorate with liver and kidney failure,(causing the yellow jaundice), abdominal pain and bleeding.
Fatality has been 50% for adults, 70% for children. No treatment is other than supportive is available.

The yellow fever vaccine can only be given through specialized clinics.

Yellow Fever must be kept refrigeratedand be given to a patient within one hour of being reconstituted.

It is a live vaccine that may cause 1-3 days of muscle aches, low-grade fever or malaise (flu symptoms), but is effective after 7-10 days. These side effects are rare and sometimes happen within the first 2 weeks after vaccination. It is not given to children under 9-12 months (unless an overwhelming need), immune compromised individuals and pregnant women. The yellow fever certificate should be kept with the traveler's passport and is valid for 10 years

Recently some very severe side effects have been noted in individuals with chronic illnesses including death. It is important that yellow fever immunization be given appropriately to the people who really do need it and also not to people who could become ill from it.

Yellow Fever links

Center for Disease Control http://www.cdc.gov/travel/diseases/yellowfever.htm

World Health Organization http://www.who.int/csr/disease/yellowfev/en/

Pan American Health Organization

http://www.paho.org/English/SHA/be_v21n2-yellowfever.htm

Innovations in the Medical Arts and Sciences that Arise During Wartime

A discussion on Dr Walter Reed and Yellow Fever

http://www.mcatmaster.com/medicine&war/yellowfever.htm