Our world is full of life. Even a tiny drop of water can hold many microorganisms. Even though they may be too tiny to be seen by us, they perform important roles that help keep the balance of our natural world. NG YI-SHENG tells us more about the different kinds of microbes.

Eukaryotes: All living things, including humans, are made up of cells. Eukaryotes are made up of several cells, linked up and working together. Some are tiny animals and plants, too small for the eye to see. The rotifer, for instance, is a water animal that is less than one millimeter long.

Protists: Protists are made up of just one cell. One famous example is Plasmodium, which causes malaria in humans. Each one is just a hundredth of a millimeter long. Another example is plankton, which grows in the sea and is eaten by whales.

Bacteria: Bacteria are also made up of single cells, but these cells are even simpler than those of protists. Some bacteria can cause diseases like tuberculosis and the plague. But, there are also good bacteria in our stomachs that help us to digest food.

Viruses: Viruses are even smaller than bacteria. Scientists aren’t sure whether they can be called life forms or not. They can’t reproduce on their own – they have to invade other cells to make more of themselves. Many illnesses are causes by viruses, such as dengue fever, the flu and AIDS.