an initiative of ACCORD

Transmission of infectious disease

How are infectious diseases transmitted?
Infectious disease can occur when disease-causing microorganisms enter the body. Another name for microorganisms that cause disease is “pathogens”. Not all microorganisms are pathogens. In fact, most micro-organisms you come into contact with on a daily basis are harmless, while others play a key role in keeping you healthy.

Pathogens can cause illness in a number of ways: by releasing poisons into the body, by preventing cells from performing their normal functions, or by multiplying rapidly and so preventing normal organ function.

Pathogens can be spread via:

  • Water. Untreated or inadequately treated water is responsible for approximately 80% of all illnesses and deaths in the developing world. (Source 1)
  • Air. Aerosols from coughs and sneezes can travel on air currents and be inhaled.
  • Direct contact with bodily fluid. Droplets of blood, respiratory tract fluid (dispersed by coughs, sneezes or talking), or sexual contact can transmit pathogens.
  • Food. Eating contaminated food can cause ingestion of pathogens.
  • Inanimate objects (or “fomites”). Indirect transmission can occur via objects such as phones, surfaces, door knobs, writing utensils and medical equipment.
  • Vectors. Living organisms can transfer pathogens from one host to another, e.g. fleas pass microorganisms from the blood of one host to another as they bite.

There are some additional recent challenges to combating the spread of infectious agents: we spend more time indoors, with more of us living in cities, working in offices and travelling – bringing us into closer contact with more people – than ever before.

But your body is not defenceless!

  • Physical barriers such as skin, hair and mucus membranes help prevent foreign microorganisms from entering your body.
  • Your immune system is able to recognise and destroy foreign microorganisms.
  • Chemical defences such as saliva, sweat, tears, mucous and stomach acid can destroy microorganisms.
  • Resident flora (“good” bacteria) live in your body and help fight other microorganisms.
  • Coughing and sneezing expel microorganisms from your lungs.
  • Vaccinations provide protection from many infectious diseases.

However, these defences can fail if pathogens are too numerous or too potent. Alternatively, the body’s natural defences may be weak. For example, people who are very young, very old, undergoing chemotherapy or suffering from AIDS generally have weak immune systems. Broken skin can also allow pathogens to enter the body.

These concepts are summarised in the diagram below.

Good hygiene prevents this step

Good hygiene is important because it prevents pathogens from entering your body in the first place.

Effective hygiene practices and proper use of products in accordance with the manufacturer’s directions are essential to maintain good standards of personal, home, public and industrial hygiene.

Click here to learn more about the regulatory systems in Australia that help ensure that hygiene products are safe for use.

Source 1: UN Press Release, 5 June 2003, “Water-related diseases responsible for 80 per cent of all illnesses, deaths in developing world”, says Secretary General in Environment Day message.

useful stuff...

Disease-causing organisms are called "pathogens".
A study showed that a virus spread from the office doorknob to 40-60% of workers and visitors within 2-4 hours!4 The healthy skin releases approximately 10 million particles a day, with 1 million of these containing viable bacteria.5 The flu virus can survive and cause human infection for 2 to 8 hours after being deposited on a surface.6
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