Viruses and bacteria are both microscopic and can both cause disease. Viruses differ from bacteria in that all viruses are obligate parasites, which means they cannot live or survive independently, on their own, but have to live inside cells.
Viruses do not have organelles essential to life such as animal and plant cell have. A virus has no nucleus, no mitochondria, no Golgi apparatus, no endoplasmic reticulum and no ribosomes, therefore it cannot perform propagate itself.
Viruses invade cells and hijack the cell machinery to manufacture their own virus components. Once assembled together, these parts form new viruses which in cases causes the cell to die.
