Viruses can only be cultured (grown for study) in the presence of living cells. To grow bacteriophages you must first start with a healthy layer or "lawn" of susceptible bacteria.
Only then can the virus be introduced the bacteria covered agar (usually in a petri dish). Each active virus infects just one bacterium, but within a few hours hundreds of bacteria will be infected and dying. The death and destruction of the bacteria leave a small but visible "hole" in the bacterial lawn.
Microbiologists use a wide variety of characterists to identify and classify viruses including: