Bacteria are microscopic organisms with the largest are only 10 micrometres long (10 millionths of a metre). Comparing to that, viruses are even smaller.
The main problems with viruses are that they can damage the host cell when they reproduce. Once the host cell is damaged, viruses can be passed out in the airways, bloodstream, and cause disease such as colds, influenza - flu, rubella, mumps, chicken pox, AIDS and etc.