The Virus Kingdom

Viruses are so tiny that they weren't seen until electron microscopes were invented.

They are the only living things that are not made of cells.

Scientists, including doctors, knew there was something even if they couldn't find it, that caused a variety of diseases. The word was first used that way by Jenner, of smallpox fame, about 1800.

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The "Virus" kingdom is one of the five primary sub-divisions of all living things.