Viruses cannot replicate without a host cell. They infect plants, animals, fungi, bacteria and blue-green algae. One can only see them with an electron microscope.
Bacteria are between 10 to 100x larger than viruses. They can be shaped like a rod (bacillus), a sphere (cocci), a spiral (spirillum) or a comma (vibrio).
Some bacteria live and multiply in the environment, while others are adapted to life within human or animal hosts. Some bacteria can double in number every 15 minutes; others take weeks or months to multiply.