Viruses are amazing. They aren't even considered living things, yet they wreak havoc on our world. They are considered activated or deactivated rather than alive or dead. They do not contain cells and they cannot live on their own.
Viruses need to be attached to tissue to thrive. While you could observe a colony of bacteria in a petri dish feasting on agar, you couldn't observe a colony of viruses that way. If you were looking at them in a dish, there would also have to be living tissue there. This is one reason why there are very few anti-viral medications available.
Consider a virus attacking your body. It becomes part of your cells in order to replicate itself. By the time you feel sick, many of your cells contain the DNA of the attacking virus. How would you kill the virus without killing your cells, too?
Of viral note in the news is the theory that the SARS (Severe Acute Respiratory Syndrome) virus became aerosolized through the plumbing of an apartment complex and in that way infected over 300 people in that complex last year. This possibility opens up new avenues of public health concern regarding disease prevention.