"And God saw everything that he had made, and, behold, it was very good."This is on the sixth day, after God has created everything (but before the Fall). Does this creation include viruses?
I'm not sure, but there is growing evidence that most (if not all) viruses deadly to humans originated in animals (and evidence that these viruses jump species through misuse and abuse of animals).
A recent article on Science Daily shows that a virus similar to the common cold (HMPV) originated some two hundred years ago in birds.