In some respects, the battle between bacteria and your body is refreshingly straightforward. Huge armies of microscopic, foreign creatures flood your body. They wreak havoc—briefly—before your better-armed defensive forces destroy them.
Viruses are a different matter. First, they’re much tinier— about a hundredth the size of
an average bacterium. In fact, viruses are so vanishingly small that even the most powerful optical microscope can’t spot one (although a cutting-edge electron microscope can). Stack viruses and bacteria together, and it’s like comparing a toddler to a brontosaurus.
To get a better feel for the difference in scale, check out www.cellsalive.com/howbig.htm. There you’ll see a simple animation that places you on the head of a pin and increases the magnification until you can spot a dust mite, a particle of pollen, and a red blood cell. Zoom in still more and you’ll be able to make out the much smaller cell of a bacterium, and then, finally, a virus.
There’s another clear difference between bacteria and viruses, but you need to step into their microscopic world to see it. Up close, a bacterium looks like a tiny alien being. It may be small (and ugly), but it’s full of life— feeding, reproducing, and generating energy with some of the same processes your own cells use. Many bacteria are even able to move by propelling themselves with long, whip-like tails, or by gliding along paths of self-produced slime.
By comparison, a virus looks more like a piece of organic debris. Its structure is simple—in fact, a virus consists of little more than a submicroscopic scrap of genetic material (either DNA or its relative, RNA) wrapped in a thin coat of protective protein. On its own, a virus is silent, inert, and completely lifeless. It’s unable to power a single one of the chemical reactions required for life.
If it weren’t for the presence of other life-forms, this is where the story would end. But as you’ll see, viruses have the uncanny ability to turn up at the right place at the right time—namely, in the midst of a normal cell’s manufacturing process.
The lifelessness of viruses makes them dangerous in another way. Because they don’t live on their own, they’re in little danger of dying naturally. In other words, they won’t run out of fuel or burn out from the hard work of cellular life. In fact, some viruses (like anthrax) can linger for years in the outside world. Others can sleep inside their hosts, waiting for the right trigger before they become active.
Source of Information : Oreilly - Your Body Missing Manual