At the end of each year, many websites, blogs, and podcasts (to name a few) summarize the past year’s events. We did the same at TWiV, naming our picks for the top 10 virology stories of 2008. Then, as the new year begins, many of the same sites predict what will happen in 2009. Such predictions are fine for areas like politics, technology, and the arts, but for virology, it’s just not possible to know what is going to happen. Why not?
The sources of viral diversity are mutation, recombination, reassortment, and selection. The interplay of these forces result in the changes in a viral population known as virus evolution. The number of all possible virus mutants is so large that it can never be tested in nature. Sequence comparisons of some RNA virus genomes have revealed that over half of the nucleotides can be changed by mutation. In other words, for a 10 kB viral RNA genome, over 45000 sequences are possible. Since there are 4135 atoms in the visible universe, this is clearly an astronomical number. With so many possibilities, it is impossible to predict the future. And this does not even take into consideration the fact that we barely understand the selective forces acting upon viral populations in nature.
The problem is best summarized in Principles of Virology: “Those who seek to predict the trajectory of virus evolution face enormous challenges, as mutation rates are probabilistic and viral quasispecies are indeterminate”.
We can make one virology prediction for 2009: that virus evolution will continue. New viral mutants will arise, and the possibilities are unfathomable. The new viruses that arise will come from those existing today, by mutation, recombination, and reassortment.