Why aren't viruses considered living things?
Viruses, like bacteria, are microscopic and cause human diseases. But unlike bacteria, viruses are acellular particles (meaning they aren't made up of living cells like plants and animals are), consisting instead of a central core of either DNA or RNA surrounded by a coating of protein.
Viruses also lack the properties of living things: They have no energy metabolism, they do not grow, they produce no waste products, and they do not respond to stimuli. They also don't reproduce independently but must replicate by invading living cells.