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All very small organisms that cannot be seen without the use of powerful microscopes are called microbes. These microscopic organisms include bacteria, fungi, protists, archaea, and viruses.

Let us take a quick look at each one of these.

Bacteria

Living things are classified in groups according to the features they have in common. The largest groups are the five kingdoms: animals, plants, fungi,, protists and monerans. Bacteria belong to a this last kingdom called monerans.

A bacterium’s body consists of a single cell. Bacteria can be of different shapes.

Most often we associate bacteria with diseases. However, the fact is without bacteria, the life on earth may not even be possible. All plants, animals and the entire ecosystem depend on bacteria.

Our body contains many types of bacteria that are found naturally and that we eat as part of our food. Digestion of the food we eat is simply because of the existence of bacteria in our body.

However, some types of bacteria are harmful to us. They invade the body cells and multiply fast enough to create a bacterial infection.


Fungi

Fungi are of different sizes and types. They are of single cell types as well as multicellular. Mushrooms for example, are a very commonly known type of fungi.

The different classes in the kingdom of fungi are: Oomycetes (rusts and mildews), Zygomycetes (molds), Asomycetes (sac fungi), Basidiomycetes (club fungi), Deuteromycetes (Fungi imperfecti) and Acrasiomycota (slime molds).

Fungi are not plants. They cannot make their own food like the plants do. Just like the bacteria, some fungi are very useful to humans. As a matter of fact, fungi are used in some vaccines to fight against several kinds of disease causing bacteria.


Protists

This is another kingdom of living things that are not plants or animals. The Protists group consists of Chrysophyta (golden algae), Sarcomastigophora (amoebas, flagellates, opalinids), Sporozoa and Ciliophora(cilates).


Archaea

Arachea may seem to be very much like bacteria, but are a different class by themselves. Their genetic structure is different from bacteria.


Viruses

A virus is so small that it's body does not even contain a single cell. Therefore a virus is called an ascellular organism. Its “body” is made up of nucleic acid – a component that allows it to replicate.

In fact, replication is the only activity a virus can perform. It does not grow or breathe or move. Also, it needs a cell in a host's body to replicate. When a virus enters a host's body, it replicates very fast and invades the normal functioning of the cell thereby causing what is called a viral infection.

Viruses do not fall into the definition of living things, as a result many scientists do not even consider them as living.