Skip main navigation

Background: What are microbes?

Article explaining what microbes are, and providing basic information about bacteria, fungi, and viruses.

Links to activities: make your own microbes, yeast races, and respiratory hygiene.


Microbes, also known as microorganisms, germs, or bugs, are tiny living organisms too small to be seen with the naked eye.

  • Microbes is the common generic term used to describe viruses, bacteria, or fungi, regardless of whether they are useful or harmful.
  • They are found almost everywhere on earth, and are an important part of life.
  • They come in many different shapes and sizes.
  • The scientifically accepted term to describe microbes which cause us to become ill is “pathogenic” or a “pathogen”. It’s important to note that this term is not well understood by the public. The Royal Society of Public Health (RSPH) poll showed that out of 2000 people, only 43% said that pathogens are “usually or always” harmful, and 35% said they did not know what the term meant.
  • The non-scientific common word for microbes is “germs”. Traditionally this term describes harmful microbes, with visual images showing them as threatening and disgusting. In recent years, it has become common to use the term to describe any type of microbe.
  • To avoid misleading children, it is very important to specify whether you are talking about harmful or non-harmful germs. The media feature articles about tests showing thousands or millions of “germs” in the toilet or on surfaces, but fail to point out that these are not always harmful and in some cases are beneficial. In the RSPH online poll only 36% of participants thought that germs were sometimes good and sometimes bad, whilst 58% thought that they were “usually or always harmful”.
  • In our course we use the term “bug” to refer to any microbe.
Did you know? The term “human microbiome” is specific to the microbes that inhabit our body (skin, mouth, gut etc…). There are trillions of microbes in our bodies!


Viruses are the smallest of the microbes and can be harmful to humans. Viruses cannot survive by themselves. They need a “host” cell to survive and reproduce. Once inside the host cell, they rapidly multiply and destroy the cell in the process.

Close up image of Rhinovirus (the common cold) One type of virus is Rhinovirus, also known as the common cold virus. There are over 250 different kinds of cold viruses! Image taken from Giant Microbes.


Fungi are the largest of the microbes, and are multi-cellular organisms. Some fungi are useful and some can be harmful to humans. Fungi obtain their food by either decomposing dead organic matter or by living as parasites on a host. Fungi can be harmful by causing infection or being poisonous to eat.

A type of useful fungus is Penicillium which produces the antibiotic penicillin. There are also fungi that can be eaten, such as Agaricus, commonly known as the white button mushroom.

Close up of Penicillium (fungus which produces penicillin) One type of fungus is Penicillium which produces the antibiotic penicillin. Image taken from CDC Public Health Image Library.


Bacteria are smaller than fungi, but larger than viruses, they can be both useful and harmful to humans. They can be divided into three main groups by their shapes – cocci (balls), bacilli (rods) and spirals. Cocci can also be broken down into three groups by how the cocci are arranged: staphylococci (clusters), streptococci (chains) and diplococci (pairs). These shapes can be used to identify the type of infection a patient has.

During their normal growth, some bacteria produce toxins which are extremely harmful to humans and cause infections. Other bacteria are completely harmless to humans and some can be extremely useful to us. Over 70% of bacteria are non-pathogenic.

Close up image of Staphylococcus aureus (bacteria) One type of bacteria is Staphylococcus aureus, which is part of normal human flora, but can sometimes cause skin infections. Image taken from Giant Microbes.

Please let us know in the comments below – do you often come across confusion around these words from children and young people?

This article is from the free online

e-Bug Health Educator Training

Created by
FutureLearn - Learning For Life

Reach your personal and professional goals

Unlock access to hundreds of expert online courses and degrees from top universities and educators to gain accredited qualifications and professional CV-building certificates.

Join over 18 million learners to launch, switch or build upon your career, all at your own pace, across a wide range of topic areas.

Start Learning now