What's the smallest living organism?
Is it a virus? Or a bacterium? Are viruses living? We could not exist without bacteria. Could we exist without viruses? These, and other questions will be addressed through the Prokaryotes and Viruses chapter.
Prokaryotes and Viruses discusses prokaryotes and viruses. Prokaryotes are the smallest living organisms, and viruses, by the definition of "living," are not actually alive. Both can be harmful, but we need bacteria to survive, and the uniqueness of viruses has made them a very useful tool to improve the human condition.