Return of the viruses
I just read a disturbing post on the amusingly-titled serious-science blog "Mystery Rays from Outer Space", discussing two examples of human pathogens that apparently escaped from laboratories. The key evidence, in each case, is evolution... or rather, lack of evolution....
The most recent example is from PLoS-One: a dengue virus now infecting people in Brazil and Columbia is almost identical, genetically, to a virus last seen in Asia twenty years ago. An earlier example was published in Nature in 1978, reporting an H1N1 flu virus whose closest genetic match was from 1950 version.
Why do we think that these viruses escaped from (or were released from) laboratories? Couldn't they have been living in some isolated human population in a remote valley, or perhaps in some wild species? No. Over a twenty-year period, the viruses would have evolved, through some combination of random mutation, nonrandom natural selection (like that imposed by the immune systems of its hosts), and random genetic drift (changes in the frequency of different genotypes due to chance). As the 1978 Nature paper put it, the only way the virus wouldn't have evolved over the course of 20 years is if it were "frozen" somewhere.
A laboratory freezer seems the most likely culprit. Under that hypothesis, scientists must have thawed an old sample, presumably for research purposes, and then carelessly infected themselves and then others. It is possible, however, that the virus was frozen somewhere else, like in the frozen body of a mountain climber recovered from a glacier after twenty years.