Microbes are organisms that one cannot see without a microscope. All bacteria and archaea are microbes.

Most protists are also microbes. Organisms such as paramecia and amoebas are protists. They like wet environments. Some fungi, such as molds and yeasts, are also microbes.

Viruses, such as the influenza virus, are also often called microbes, even though they are not technically living organisms.

The traditional categorization of living organisms. Prokaryotic organisms (bacteria and archaea) do not have nuclei in their cells. Protists, plants, animals, and fungi are all eukaryotic: their cells have nuclei. 

Small microbes dry up easily. Because of this, they require a wet environment to survive. Water also makes it possible for them to move from one place to another. This is why most microbes live in the water.

Phytoplankton and cyanobacteria are significant producers in water ecosystems. They use sunlight to produce chemical energy. This energy is then transferred up the food chain to consumers. This happens for example when a water flea eats a small alga.

Many microbes function as decomposers in the soil. Decomposers free up nutrients that are bound to dead organisms or their fecal matter and release these nutrients back to the environment, where they are then used by other organisms.

Some microbes are pathogens. This means that they cause diseases. For example, drinking dirty water result in diarrhea caused by a microbe.

Microbes were the first organisms on the planet. Over time, unicellular bacteria developed into more complex multicellular organisms that had specialized cells for different functions. Multicellularity also made it possible for organisms to develop structures that protected them from drought, which in turn made it possible for life to move from water to land. Photosynthetic bacteria and unicellular algae also produced oxygen that formed a planetary atmosphere that could sustain more complex forms of life.