Not every illness is catching, and not every microorganism is out to get you. This page sorts out which is which — and then looks at two doctors who worked out how to stop a disease spreading long before anyone knew what a bacterium was.
📘 What you need to know
A disease is an illness or disorder of the body or mind that leads to poor health, and comes with a set of signs and symptoms.
A pathogen is any microorganism that causes disease in another organism.
Four groups contain pathogens: bacteria, fungi, protists and viruses. No archaea are known to cause disease in humans.
Most bacteria, fungi and protists are harmless. All viruses are pathogenic, because they can only exist inside living cells.
Pathogens cause communicable (infectious) diseases, which pass from an infected host to a healthy one.
Non-communicable diseases — cancer, cardiovascular disease, malnutrition — are not passed on.
Careful observation has driven major breakthroughs: John Snow and cholera, Semmelweis and childbed fever.
What counts as a pathogen
Your body is covered and filled with microorganisms right now, and almost none of them are doing you any harm. Some are actively useful. A microorganism only earns the name pathogen if it causes disease in another organism.
Being a bacterium does not make something dangerous. Being a virus does — and the reason is in how a virus makes copies of itself.
If you are asked why all viruses are pathogens but only some bacteria are, the answer is about lifestyle, not nastiness. A virus can only reproduce by taking over a living cell, which damages the host. A bacterium can often just live quietly on its own.
Communicable vs non-communicable
The dividing line is simple: can it pass from one person to another?
Type
What it means
Examples
Communicable (infectious)
Caused by pathogens; transmitted from an infected host to a healthy one. Some also affect animals and pass to humans.
Cholera, malaria, HIV/AIDS, tuberculosis
Non-communicable (non-infectious)
Long-term, often degenerative; not caused by pathogens. Includes inherited conditions, deficiency diseases and mental illness.
“Communicable” and “infectious” and “transmissible” all mean the same thing here. Exam papers use whichever they feel like, so recognise all three.
Careful observation changes everything
Two nineteenth-century stories show what you can achieve by watching closely, even without knowing what the pathogen is. Both are on the syllabus as examples of how science actually works.
John Snow and cholera
Cholera is a water- and food-borne disease caused by the bacterium Vibrio cholerae. Infected people pass huge numbers of the bacteria in their faeces. If those faeces get into the water supply, or if an infected person handles food without washing their hands, the bacteria are passed on. The disease is common wherever people lack clean water and safe food.
🧩 How one doctor stopped an outbreak
In 1854 a cholera outbreak in Soho, London killed over 500 people in a month.
John Snow had already seen the disease in a mining village in 1832, so he knew its symptoms and how it seemed to spread.
He mapped the cases and traced them back to a single water pump.
The pump handle was removed. The outbreak ended.
It later turned out the pump stood a few feet from a cesspit contaminated with Vibrio cholerae.
His evidence fed into the germ theory of disease and transformed sanitation.
Public health still works this way. Find the link you can cut most easily — clean water, hand washing, a vaccine — and cut it.
Semmelweis and childbed fever
Puerperal fever, or childbed fever, is a bacterial infection of the female reproductive tract after childbirth, transmitted by direct contact during delivery. In the nineteenth century it was the most common cause of death linked to childbirth.
The Hungarian physician Ignaz Semmelweis noticed that one maternity ward had far more deaths than another. Looking closer, he found that the doctors and medical students in that ward went straight from carrying out autopsies to delivering babies, without washing their hands.
He found a correlation between the number of autopsies and the number of deaths, and suggested that particles were being carried from the corpses to the women. He made hand washing compulsory, and later washing of instruments too. Deaths fell sharply. Routine hand washing in hospitals dates from this work.
Notice the word correlation. Semmelweis did not prove the cause — germs had not been discovered. He spotted a pattern, proposed a mechanism and tested it by changing behaviour. That sequence, observation to hypothesis to test, is what examiners mean by “the nature of science”.
Worked examples
WORKED EXAMPLE
Explain why all viruses are considered pathogenic, while many bacteria are not. [3]
Start with what a virus can doA virus can only exist by getting inside the living cells of another organism.Say what that does to the hostIt uses the host cell to make more viruses, which damages or destroys that cell, so disease results.Now contrastMany bacteria live independently and cause no harm, so only some species are pathogens.Viruses must infect to exist; bacteria do notthe comparison mark needs the word “bacteria” written explicitly
WORKED EXAMPLE
Suggest how John Snow was able to control the Soho outbreak without knowing what caused cholera. [3]
His prior experienceHe had seen cholera before, so he recognised the symptoms and had ideas about how it spread.The observationHe mapped the cases and found they all traced back to one water pump.The actionRemoving the pump handle broke the route of transmission, so no new people were exposed and the outbreak stopped.Identify the route, then cut it“suggest” questions reward reasoning, so explain why removing the handle worked
💡 Exam tip
Learn one named example of a disease for each pathogen group. A named example is frequently worth a mark on its own.
Say microorganism that causes disease when defining a pathogen. “A germ” or “something bad” will not score.
Remember the archaea point — it is an easy multiple-choice trap.
For the nature-of-science examples, learn the observation, the action taken and the outcome. Three points, three marks.
Correlation is not cause. If a question gives you data, say “these are correlated” rather than “this proves”.
⚠ Common mix-up
Thinking all bacteria are pathogens. Most are harmless and some are essential to you.
Calling cancer infectious. It is non-communicable — no pathogen is involved.
Saying a virus is a cell. It is not. It has no cytoplasm, no ribosomes and no metabolism of its own.
Writing that Snow discovered the cholera bacterium. He did not. He identified the route of transmission.
Confusing symptoms with signs. Symptoms are what the patient feels; signs are what can be measured or seen.
Up next: Pathogen Barriers: Skin & Mucous Membranes — the defences that stop most pathogens before the immune system even hears about them.
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