(3) How are pathogens spread?

There many sorts of ways that pathogens can spread.

Air

Pathogens are carried in air currents and breathed in e.g. fungal spores.

Airborne pathogens are conveyed through the air in water droplets when we cough or sneeze e.g. the influenza virus giving us 'flue' or tuberculosis.

When people are crowded together, their close proximity allows an easy transfer of a communicable disease from one person to another e.g. coughing out a cold or flue virus!

 

Direct contact and body fluids

Pathogens can be picked up by merely touching a contaminated surface including someone's skin e.g. athlete's foot is a fungus which makes skin itch and flake off.

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

Sexually transmitted infections/diseases (STI/STD) are passed on in sexual activity where bodies are in direct contact.

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

Pathogens like Ebola can be spread through contact with infected bodily fluids of another person.

These include like blood (drug users sharing needles), vomit, saliva, faeces, breast feeding milk (mother to child), and sexual activity (contact with semen). HIV is a good example.

 

Water

Dirty contaminated water is a common source of pathogens and should not be drank or bathed in. Cholera is a bacterial infection spread by drinking water contaminated with the diarrhoea of people already infected with the bacteria.

 

Animal vectors

Some pathogens can be carried and transported by an animal organisms referred to as vectors.

The mosquito is an example of an animal vector which carries the protist pathogen that causes malaria.

 

Soil

See plant disease notes - pathogens in the soil

 

Food

Pathogens maybe present in food.

Contaminated food may contain the healthy3comdisease-04.htm bacterium causing food poisoning.

The Helicobacter pylori bacterium that causes stomach ulcers maybe found in contaminated food or water.

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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 how pathogens are spread

How Are Pathogens Spread?

1. What Is Transmission?

• Transmission is the process by which pathogens are passed from one host to another.

• It’s how communicable (infectious) diseases spread between individuals, animals, or even plants.

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2. Main Methods of Pathogen Transmission

A. Direct Contact

• Touching infected individuals or contaminated surfaces.

• Includes sexual contact, cuts, or skin-to-skin contact.

• Examples: Athlete’s foot, STIs like gonorrhoea.

B. Airborne Transmission

• Pathogens are carried in droplets from coughs, sneezes, or even talking.

• Droplets can be inhaled or settle on surfaces.

• Examples: Influenza, COVID-19, tuberculosis.

C. Waterborne Transmission

• Drinking or coming into contact with contaminated water.

• Often linked to poor sanitation.

• Examples: Cholera, typhoid.

D. Foodborne Transmission

• Eating contaminated or undercooked food.

• Poor hygiene during food preparation can spread bacteria.

• Examples: Salmonella, E. coli.

E. Vector Transmission

• A vector is an organism that carries pathogens between hosts.

• Vectors are not usually affected by the disease themselves.

• Examples:

  • Mosquitoes (malaria)

  • Ticks (Lyme disease)

  • Houseflies (dysentery)

F. Body Fluids

• Spread through blood, saliva, semen, or breast milk.

• Often involves needle sharing, blood transfusions, or unprotected sex.

• Examples: HIV, hepatitis B.

G. Contaminated Surfaces (Fomites)

• Pathogens can survive on surfaces like doorknobs, phones, or desks.

• Touching these and then your face can lead to infection.

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Why Is This Important in Human Biology?

• Helps us understand how to prevent and control disease outbreaks.

• Supports knowledge of immune system responses and public health strategies.

• Explains why vaccination, hygiene, and sanitation are essential.

• Encourages scientific literacy — knowing how diseases spread helps reduce fear and misinformation.

• Links to real-world issues like pandemics, antibiotic resistance, and global health.

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Quick Summary Table

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Keywords, phrases and learning objectives for this part on how communicable diseases spread

How are pathogens causing communicable diseases infections spread? ways of transfer by body fluids food animal vectors air airborne water soil bacteria fungi protists microorganisms viruses

Be able to describe how pathogens are spread, including:

• a) in water, including cholera bacterium

  • You can be infected with a pathogen by coming into contact with contaminated water - which is why swimming bath waters are treated to kill bacteria with chlorine or ozone. In poor third world countries the bacterial infection cholera, which causes diarrhoea and dehydration, is readily spread in water contaminated with the faeces of cholera sufferers. It is potentially very serious, particularly for the very young and the very old and undernourished adults and children in poor third world countries with poor sanitation.

• b) by food, including Salmonella bacterium infection

  • If you eat food contaminated with pathogens the resulting food poisoning effects can be very unpleasant and potentially very serious, particularly for the very young and the very old and the poor of the third world. If food is kept too long at the wrong temperature, left out in the open, or food like meat undercooked, you may be poisoned by the bacterium salmonella.

• c) airborne (eg coughing, sneezing), including influenza virus (causes flue)

  • If you are suffering from a cough, chest infection or flue etc. and you don't take precautions with a large handkerchief or tissue, when you cough or sneeze you blast out into the air a fine mist of water droplets containing millions of bacteria or viruses. People around you breathe in you exhaled pathogens and potentially become infected. Lots of people in a crowded room are great breeding places for pathogens!

• d) by contact, including athlete’s foot fungus infection

  • You can be infected with a pathogen just by touching a contaminated surface with e.g. your hand or foot. A common example is the spread of athlete's foot, a fungal infection easily spread in swimming bath surfaces, shower floors, towels i.e. anything an athlete's foot carrier has been in contact with.

• e) by body fluids, including HIV infection

  • The HIV virus causes AIDS, a disease that stops our immune system from functioning properly - you become more susceptible to infectious diseases than a normal healthy person and the condition is often fatal in the end, despite the best efforts of anti-viral drugs. These kinds of pathogens can only be passed on by direct contact with body fluids from another person e.g. from a HIV carrier's sperm during sexual intercourse, or some body penetrating situation e.g. using the same drug needle as a HIV carrier.

• f) by animal vectors (animals that spread diseases), including:

  • (i) housefly: dysentery bacterium

    • The common housefly is a carrier of a nasty protozoan bacterium. This pathogen causes dysentery, a disease that expresses itself with severe diarrhoea and dehydration. Again this can have serious consequences for the very young,  the very old and the poor of the third world.

  • (ii) Anopheles mosquito: malarial protozoan

    • The mosquito is a carrier of protozoan pathogen that causes the disease called malaria, a disease that causes potentially fatal kidney and brain damage. This serious infectious disease is passed onto another animal which is bitten by a mosquito - a mosquito bite is a bit more serious than a bee or wasp sting!

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