What is the difference between bacteria and viruses?

 Bacteria are simple cells without a nucleus, called prokaryotes. They contain DNA, RNA, have organelles that allow them to process food, are able to self-replicate, and are able to move about their environment through small hair or whip like things on their cell membrane. However, they lack the more advanced organelles of plant and animal cells. They are the simplest form of life that we know of.

Bacteria do damage because they'll either consume their host, or their waste is toxic to the host. They also reproduce very quickly. But they can also be killed by materials toxic to them but not the host.

Viruses are just DNA or RNA surrounded by a protein shell. They sit on the edge of being alive. They can't process food, move about on their own, or self-replicate. In order to reproduce, they have to hijack a living cell. They inject their genetic payload into the host cell, which tells it to start making copies of the virus.

The host cell will either starve itself to death, or burst when it becomes too full of new viruses. On some occasions, the genetic payload can damage the host cell's DNA and cause it to become cancerous.

Viruses are very hard to kill with medicine. They're incredibly small and don't have much to them. You can't really poison something that isn't really alive. Best we can do is provide the host's immune system with a wanted poster. So that when it sees the virus, it immediately recognizes it as being harmful. That's what vaccines do.