How are viruses and human cells related?

There are a number of similarities between viruses and cells. Both are too small to be seen with naked eyes and require a microscope for observation. Both contain genetic material, in the form of DNA and/or RNA. Both of them can replicate, that is, produce more organisms similar to themselves. Both of them are capable of causing diseases to other life forms.

one would be wind another would be water density

when it is at an intermediate population density