A microorganism is a living organism that is so tiny it can only be seen with the aid of a microscope.
Consider that the largest human cells are about the diameter of a human hair. Bacteria cells are one-hundredth the size of a human cell and viruses are much, much smaller again. If you imagine that a single virus was the size of an adult human, a bacterium (single cell) would be over 10 storeys high by comparison. A bacterium next to a human cell is like a tiny boat next to a big ship. Hundreds of thousands of bacteria could fit onto a pinhead.
The main microorganisms in and on our bodies are protozoa, algae, fungi, bacteria and viruses. Most microorganisms are beneficial, for example, there are microorganisms in our large intestine that synthesise vitamins and allow them to be absorbed into the bloodstream. However, a tiny minority are pathogens (disease-causing agents). These pathogens, often called germs or bugs, are a threat to all life forms.