Oh. Well, the same questions apply. What did it evolve from? Why did it evolve into its present form? What will take its place to the job it presently does when it evolves into whatever it evolves into? What will it evolve into?
“The bacterium evolved.”
How do you figure? The bacterium found a host, and infected it, killing off those in the host population who could not tolerate the infection. How exactly does this change the bacteria into something fundamentally different?