Measles

Ian York, an assistant professor in the Department of Microbiology and Molecular Genetics at Michigan State University, has a series of postings on the disease of measles and the mystery of why it became less virulent:

Measles week, part I: Introduction
Measles week, part II: Emerging disease
Measles week, Part III: Not the answers
Measles week, part IV: Some of the answers
Measles week, Part V: What about the vaccine?

Here's one tidbit to try to tease you to read the above:

So, based on genomic information, written documentation, and linguistic evidence, measles isn’t an ancient disease; it’s a disease that jumped into humans some time in the first millennium A.D.