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Communicable diseases: 3. How are pathogens - infections spread? The ways in which infectious diseases are transferred?

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Sub-index of biology notes on communicable diseases

(3) How are pathogens spread?

There many sorts of ways that pathogens can spread.


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.


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.


See plant disease notes - pathogens in the soil


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.

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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