A virus is a set of pieces of protein, which does absolutely nothing on its own. Once it gets into your cells, it takes over the cellular machinery, forcing your cell to make duplicate virii until there are so many the cell bursts. This is how they inflict damage. Bacteria are independent cells. Many bacteria do not need a host for any reason, and are not harmful. Those bacteria that are dangerous produce some toxic substance, either as a byproduct of their digestion process or as part of their cell wall. A virus is smaller, which is why some precautions are ineffective; antibiotics only work on bacteria, because they are targeted at the bacterium's cell wall.