First of all, bacteria are living organisms. The classification of viruses as living things, on the other hand, is debatable; though they are usually considered as non-living organisms as they lack some key attributes of living organisms.
For instance, viruses are not made up of cells. Cells are regarded as the fundamental units of life (Cell Theory). A virus is simply made up of a nucleic acid wrapped by a protective coating, a protein called a capsid. A bacteria, on the other hand, which is a living thing, is a unicellular organism.
A living organism's genetic code is encoded in its DNA. This DNA is replicated during growth and reproduction in a process called replication. DNA can be translated to RNA through transcription. This process is necessary as it helps "read" RNA. RNA then is then used to produce proteins in a process referred to as translation. This happens in a bacteria. However, viruses only contain either a DNA or an RNA. This is the reason why it needs a host. It needs to incorporate its DNA/RNA to the host's DNA so that it hi-jacks the host's cell's mechanism and use it to its advantage.
Bacteria can reproduce (another key characteristic of living things) through an asexual method referred to as fission. Viruses, on the other hand, cannot do this. Like what was mentioned in the previous paragraph, viruses takes advantage of the host's mechanism to replicate.
When it comes to size, bacteria are usually larger (1000nm) since they are unicellular, and hence consist of other organelles of a basic cell. Viruses are only about 20 - 400nm in size.
Both a bacterium and a virus can cause diseases. However, there are differences on the diseases they cause. Viral disease are systematic while bacterial disease are usually localized. When it comes to treatments, on the other hand, diseases caused by bacteria can usually be treated using antibiotics. Disease caused by viruses, on the other hand, cannot be cured. Vaccines merely prevent the spread or slow down reproduction but not stop it completely.
In summary, we can see that viruses merely "mimic" living organisms, but are, in fact, non-living things using the mechanisms of a host cell. Bacteria, on the other hand, are living organisms, and thus posses all characteristics of living things:
- They are composed of cells. viruses are merely capsids.
- They are organized.
- They use energy (metabolism) for growth. Viruses simply rely on their host.
- They respond to their environment - constitute a complex set of behavior.
- They grow. Viruses are merely a vessel with DNA/RNA. They don't have to grow.
- They reproduce. Viruses can't do this without the host.
- They adapt to their environment. Viruses may undergo mutation; but this adaptation is different in many waiys.