The disease is most commonly passed to humans through bites from fleas that have fed on the domestic rat, Rattus rattus. This was not fully realized until 1914. Today the disease is not common but small outbreaks can still occur in crowded, unsanitary situations where the rat populations are not controlled. Bacteria are living, single-celled organisms. Viruses are much smaller than bacteria and are not living organisms in the generally accepted sense.
Viruses are collections of protein molecules without the cell membrane, protoplasm, nucleus etc.
When a virus invades living tissue it is able to replicate itself and to affect its host in ways that are recognized as illness or disease. Outside its living host a virus is completely inert. Not all viruses cause disease but many do. Viruses are not affected by antibiotics. Fies can be disastrous.