Here's a general size comparison:

* Human cells: 10-100 micrometers (µm) in diameter

* Bacteria: 0.5-10 µm in diameter

* Viruses: 20-400 nanometers (nm) in diameter (1 µm = 1000 nm)

As you can see, viruses are about 10 to 100 times smaller than bacteria and 100 to 1000 times smaller than human cells.

Think of it this way: If a human cell were the size of a basketball, a bacterium would be about the size of a grape, and a virus would be about the size of a grain of sand.