CHANGE: Diseases can rapidly evolve to become more—or less—virulent, according to songbird study. “There’s an expectation that a very virulent disease like this one will become milder over time, to improve its ability to spread. Otherwise, it just kills the host and that’s the end of it for the organism. House Finch eye disease gave us an opportunity to test this—and we were surprised to see it actually become worse rather than milder.”