By using the electron microscope. it has been shown that viruses range in
size from about 300 millimicrons to 10 millimicrons. What is a millimicron'? It is
one thousandth of a micron. And a micron is about 0.00004 millimeters.
Nobody is quite sure yet exactly what viruses are. Some scientists think
they are closely related to bacteria. Others believe they are like elementary particles,
similar to “genes.” Still others believe viruses are possibly halfway between living
and non-living matter.
As far as we know, viruses can grow and reproduce only in living tissue.
This means it's impossible to cultivate them away from living tissue. and makes it
difficult to study their growth habits. That‘s why they are classified by their ability
to infect living cells and by the reactions they produce in the body of animals or
humans.