(2) Which types of pathogens cause communicable diseases?

The spreading, prevention and treatment of pathogens including:

bacteria - chlamydia, salmonella, tuberculosis,

viruses - HIV, leading to AIDS, colds, flue and human papilloma virus (HPV),

fungi - athletes foot and potato blight

 

Communicable diseases are caused by pathogens - types of microorganisms that enter an organism e.g. your body, and cause disease.

Pathogens can infect and cause diseases in both plants and animals, AND can spread communicable diseases from one organism to another.

 

Types of pathogen that cause disease

Bacteria

Bacteria are very small cells, compared to your own body cells, which can rapidly reproduce by cell division in your body.

They make you feel ill by damaging your body's cells and tissues and producing toxins - poisons produced as a by-product of the bacteria's cell chemistry that can damage your cells and tissues.

You should note that many bacteria are harmless and some are useful and some essential for a healthy body. Bacterial biochemistry makes cheese and yoghurt. Bacteria break down waste and they are very involved in the digestion of food, absorption of essential nutrients and are much needed for a healthy gut. There are 300-500 types of bacteria in your gut that work for your benefit along with viruses and fungi.

Unfortunately some bacteria are harmful pathogens to both plants and animals.

 

Viruses  (See also Viruses - structure, reproduction and cell destruction)

Viruses are NOT cells, they are much smaller than bacteria and damage the cells in which they reproduce.

Viruses rapidly replicate by invading a cell and using the cell's genetic machinery to reproduce themselves i.e. copies of the original virus.

The virus 'invaded' cell then bursts releasing lots of new viruses to go and invade other cells.

The cell damage makes you feel ill as your body fights back to make as many good cells as it can, to replace those destroyed by the virus.

 

Protists

Protists are all eukaryotes, often single-celled and there are many types and sizes of them.

Some eukaryotic protists are parasites and live on or inside the 'host' organism causing some kind of damage.

They are usually transferred to an organism by a vector which isn't affected by the disease itself e.g. an insect carrying protist e.g. malaria is caused by an insect (e.g. mosquito) that carries the protist.

 

Fungi

Fungi can be single celled or others have a 'body' consisting of multi-celled thread-like structure called hyphae.

Hyphae can grow and penetrate human skin and the surface of plants causing damage.

Hyphae can produce spores that spread to other plants and animals.

Athlete's foot is a fungus which makes skin itch and flake off.

It is most commonly spreading by a person coming into contact with a surface touched by an infected person e.g. shower floors or towels.

 

Larger organisms - parasites - some nasty species out there!

Helminths are a type of parasitic worm that can get inside your body e.g. tapeworms, flukes, and roundworms. Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths.

Soil-transmitted helminthiases are responsible for parasitic infections in as much as a quarter of the human population worldwide. They can cause damage to the intestine wall, inflammation, damage organs such as the skin, lungs and live and also cause neurological problems.

Trichinosis is a disease caused by trichinae helminth, typically from infected meat, characterized by digestive disturbance, fever, and muscular rigidity.

Ironically, it is thought that infection by trichinae may reduce the development of some autoimmune diseases like Crohn's disease.

Autoimmune diseases are diseases caused by the body's immune system treating its own cells as if they are foreign and attacking them - a sort of overreaction.

 

How do we become infected with a pathogen (microorganisms or viruses)?

There are many ways in which we, as humans, can become infected with a pathogen e.g.

Pathogen entering our body through the skin when bitten by an insect or using an infected needle.

Sexually transmitted disease, when the pathogen is transferred in sexual activity - through contact of the fluids from infected reproductive organs.

(A sexually transmitted infection can be defined as an infection that is transmitted via body fluids through sexual contact.)

Pathogens breathed in through nose or mouth - aerosol droplets from somebody coughing.

Pathogens taken in through the mouth contained in contaminated food or drink (polluted water).

Pathogens on a contaminated surface e.g. lack of kitchen cleanliness or careful personal hygiene,

 

A brief note on symptoms of human infection (just some preliminary ideas)

The activity of the pathogen produces characteristic symptoms which arise from typically two situations.

(a) Reproducing bacteria produce toxins that are harmful to tissues and makes us feel unwell.

(b) Viruses invade cells and reproduce inside them, damaging them and killing them - this puts your immune system and healthy cell reproduction under strain.

These lead to having e.g. a feverish high temperature, nausea, headaches and rashes on your skin.

More details on individual pathogen pages.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points on pathogens that can cause a communicable disease

Types of Pathogens That Cause Communicable Diseases

1. What Are Pathogens?

• Pathogens are microorganisms that cause communicable (infectious) diseases.

• They can infect humans, animals, and plants.

• The four main types of pathogens are:

  • Bacteria

  • Viruses

  • Fungi

  • Protists

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2. Bacteria

• Structure: Single-celled prokaryotic organisms (no nucleus).

• Size: Larger than viruses, but still microscopic.

• How they cause disease:

  • Reproduce rapidly inside the body.

  • Release toxins that damage tissues and make us feel ill.

• Examples:

  • Salmonella – causes food poisoning.

  • Mycobacterium tuberculosis – causes tuberculosis (TB).

• Treatment: Usually with antibiotics.

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3. Viruses

• Structure: Not cells – made of genetic material (DNA or RNA) inside a protein coat.

• Size: Much smaller than bacteria.

• How they cause disease:

  • Invade host cells and use them to replicate.

  • Eventually burst the host cells, damaging tissues.

• Examples:

  • HIV – causes AIDS.

  • Influenza virus – causes flu.

  • Coronavirus – causes COVID-19.

• Treatment: Antiviral drugs (limited), vaccination is key.

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4. Fungi

• Structure: Can be unicellular (yeast) or multicellular with thread-like structures called hyphae.

• How they cause disease:

  • Grow on or inside the body.

  • Produce spores and digest tissues, causing damage.

• Examples:

  • Athlete’s foot – itchy, scaly skin.

  • Ringworm – circular rash on skin.

• Treatment: Antifungal creams or tablets.

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5. Protists

• Structure: Single-celled eukaryotes (have a nucleus).

• How they cause disease:

  • Often parasites – live in or on a host.

  • Transmitted by vectors (e.g. mosquitoes).

• Examples:

  • Plasmodium – causes malaria, spread by mosquitoes.

• Treatment: Antiprotozoal drugs, vector control (e.g. nets, insecticides).

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Why Is This Important in Human Biology?

• Helps us understand how different pathogens affect the body.

• Explains why different treatments are needed (e.g. antibiotics don’t work on viruses).

• Supports knowledge of disease prevention, like hygiene, vaccination, and vector control.

• Links to immune system responses and how the body defends itself.

• Essential for understanding public health, epidemics, and global disease control.

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Quick Comparison Table

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Keywords, phrases and learning objectives for this part on the types of pathogens causing communicable diseases

Know and be able to describe the types of pathogens that can cause a communicable disease e.g. microorganisms like bacteria, fungi, protists and viruses and parasites too.

Know ways in which we can become infected.

Know some typical symptoms of human infections.

• Know that infectious diseases are caused by pathogens.

• An infectious disease is one that spreads from one person to another.

• Microorganisms that cause infectious disease are called pathogens.

• Bacteria and viruses may reproduce rapidly inside the body and produce poisons (toxins) that make us feel ill.

• Bacteria and certain protozoa are very small cells which can rapidly reproduce by cell division in your body making you feel ill by damaging your body's cells and producing toxins (poisons produced as a by-product of the bacteria's cell chemistry).

• Viruses are NOT cells and much smaller than bacteria and damage the cells in which they reproduce.

• Viruses replicate by invading a cell and using the cell's genetic machinery to reproduce themselves i.e. copies of the original virus.

• The virus 'invaded' cell then bursts releasing lots of new viruses.

• Fungi are also pathogens and includes microorganisms like yeasts and moulds (so don't eat mouldy food!).

• (Knowledge of the structure of bacteria and viruses is not required here.)

• Fungi are also pathogens and includes microorganisms like yeasts and moulds.

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