Well the bacteria need agar because its their food.
Viruses need living cells as their Food. If its a Phage then thats simple. Just get some bacterias and infect them with the Phage.
Wait for plaques to form and then harvest the plaques. Purification however involves alot of steps (14 if i remember correctly)
but that is it actually. For Eukaryotic viruses its quite similar.
Infect the cells, look for plaques and harvest them.
