Because viruses have the ability to infect cells. E.G a dendritic cell. So the only thing that can kill then are natural killer cells but then viruses reproduct in these cells
The cells were first observed using a compound microscope.
This microscope was used by Robert Hooke in the year 1665 who observed thin slices of cork under the compound microscope and saw structures which resembled small rooms and thus he named them as cells.
In reality what he actually observed were the dead plant cells.