If you mean biological viruses, it's not just possible, it's certain. In the two seconds it took you to read this, hundreds or thousands of viruses appeared that didn't exist before. Mutation happens at a very high rate, and there are a lot of viruses in the world.
If you mean computer viruses, it's not just possible, it's certain. Just that the rate is several orders of magnitude slower because it depends on human ingenuity :).