Yellow fever is a severe infectious disease, caused by a virus called a "flavivirus." This flavivirus has caused outbreaks of epidemic proportions throughout Africa and tropical America. The first written evidence of such an epidemic occurred in the Yucatan in 1648. Since that time, much has been learned about the interesting transmission patterns of this devastating illness.

In order to understand how yellow fever is passed, several terms need to be defined. The word "host" refers to an animal that can be infected with a particular disease. The term "vector" refers to an organism which can carry a particular disease-causing agent (such as a virus or bacteria) without actually developing the disease. The vector can then pass the virus or bacteria on to a new host.

Many of the common illnesses in the United States (including the common cold, many viral causes of diarrhea, and influenza or...