The population density is the measurement of the population present in an unit area. In this case in a square kilometer, 200 deer are present; it is a representation of the population density of the deer in that particular region. In ecology the population density is used to keep a record of the density and distribution of species present in an area at a particular time.
The main difference between cells and viruses is that viruses need another cell to infect so that they can reproduce. Cells on the other hand can reproduce on their own.
