One of the things I have always been curious about is why vaccines are effective for certain viruses and not others. The major viral scourge of today is, of course, HIV, with influenza probably a close second. Yellow fever was a major problem in the world until the early 1930s. In the case of HIV, a vaccine has not yet been created, despite an investment of hundreds of millions of dollars. An effective yellow fever vaccine was developed some 75 years ago, in the early 1930s. In fact, since then, nearly 300 000 000 doses of the latter vaccine have been administered without adverse effect. The question I am going to ask in this essay is, why has there been a vaccine success for one of these viruses, but not the other? In a curious way, the answer is decidedly ecological.

The primary sources for my information came from several very good virology books, discussions with a virologist colleague, plus some literature searches in our library. The first tome I used was Topley and Wilson's Volume I of Microbiology and Microbial Infections: Virology, edited by Brian Mahy and Leslie Collier (1998). A second was a popular general ecology textbook, Evolutionary Analysis, written by Scott Freeman and Jon Herron (2004). I also had a series of very productive discussions with Pat Lord, a very solid virologist in our Biology Department here at Wake Forest University.