Biologists - those who study Life and know it best - do not agree on a definition of Life. There are a few characteristics upon which people generally agree:
Homeostasis
Response to environment
Reproduction
Energy utilization
Growth and Development
Some biologists also include in their definition such features as Cells and Metabolism.
That being said, there are plenty of examples of things that are very difficult to define as being alive or not. Viruses and Prions are clear examples; they are non-cellular and they have no metabolism and it's questionable whether they grow and develop, yet it also makes some sense to consider them alive because of their stunning abilities to reproduce and respond to their environments. Epidemiologically, it absolutely makes sense to treat viruses as living entities that can be "killed". Some biologists choose to think of viruses as being alive, others don't.
Or take the example of a chicken egg. Let's say it was laid by a hen that has a rooster around, and that she is feeling broody and she's setting on it. Is it alive? Yes, it's probably easy to say that it's alive, as it will likely develop into a chick. But what if there's no rooster around? An unfertilized egg is a single cell that has a metabolism. If the hen is feeling broody, she will continue to set on it and keep it warm. But is it alive? Or what if the hen is not feeling broody and she simply lays the egg and leaves it in the box. I think it's pretty difficult to call it alive then - but it certainly was once a living cell in the hen's body. When did it cross the line into non-living status? Can it be called "dead"? Despite the apparent silliness of a chicken egg, I think it's actually pretty difficult to come up with a clear answer. In the end, we have to say "it depends". It depends both upon the circumstances and upon how we choose to define Life.
Life turns out to be too varied and complex a phenomenon to admit a simple definition.
Biology is like that - full of subtle distinctions and maddening exceptions and messy circumstances.
