How infectious diseases spread

Infectious diseases are caused by organisms (germs) such as bacteria, viruses, fungi or parasites. Germs are a part of everyday life and are found in the air, soil, water and in and on our bodies.

Some germs are helpful and even help us to stay healthy, while others are harmful and cause infection. By understanding about how people can catch an infectious disease, you can then take effective action to prevent their spread. 

Germs get into the body through the:

  • mouth – including eating, drinking or breathing
  • skin - through cuts and grazes
  • eyes
  • genitals.

Germs can spread:

  • from person to person:
    • directly through close contact  
    • indirectly from an infected person to an object (such as door handles, bench tops, food) and then to another person who comes into contact with the contaminated item
  • from a contaminated environmental source, such as an animal or the soil, to a susceptible person.

Person to person spread

This is the most common way that we get an infectious disease. Germs can spread from person to person through:

  • the air as droplets or aerosol particles
  • faecal-oral spread
  • blood or other body fluids
  • skin or mucous membrane contact
  • sexual contact.

Some infections can be spread in more than one way.

Through the air as droplets or aerosol particles (airborne)

Some infections are spread when an infected person talks, coughs or sneezes and the small droplets they produce contain germs. The droplets travel a short distance before falling. The droplets may be breathed in by people who are near, or may fall and contaminate an object or surface. Spread can also occur by touching the nose or mouth with hands contaminated by the droplets.  Examples of diseases spread by droplet:

  • common cold
  • influenza (the flu)
  • COVID-19.

Other infections are spread when an infected person talks, breathes, coughs or sneezes tiny particles that contain germs into the air. These are called small particle aerosols. Since these aerosol particles are tiny, they can stay suspended in the air for hours and be breathed in by other people. Examples of aerosol spread:

  • chickenpox
  • measles
  • TB.

Some germs can be spread by both droplets and aerosols eg, the flu.

Through faecal-oral spread

Some infections are spread when tiny amounts of faeces (poo) from an infected person are taken in by another person by their mouth. The germs may be passed directly from infected hands to the mouth or indirectly through objects, surfaces, food or water contaminated with poo. Examples of diseases spread this way:

  • campylobacter
  • giardia
  • hepatitis A.

Through blood or other body fluids

Some infections are spread when body fluids such as blood, saliva, urine (wees), faeces (poos) or semen come into direct contact with an uninfected person through kissing, sexual contact or through a needlestick injury. Examples of diseases spread through body fluids:

  • hepatitis B
  • hepatitis C
  • HIV.

Through contact with skin or mucous membrane

Some infections are spread directly when skin or mucous membrane (the thin lining of parts of the body such as nose, mouth, genitals) comes into contact with the skin or mucous membrane of an infected person. Infections may be spread indirectly when the skin comes in contact with a contaminated object. Examples of diseases spread this way:

  • head lice
  • conjunctivitis
  • ringworm.

Through sexual contact

Sexually transmitted infections (STIs) are most commonly transmitted by sexual contact. This means through vaginal, anal or oral sex.

Examples of sexually transmitted infections are:

  • chlamydia
  • gonorrhoea
  • syphilis.

Environmental spread

Some diseases are not passed on from person to person, but through contact with other environmental sources such as food, water, animals or soil.

Through contact with contaminated food or water

These diseases come from eating or drinking food or water contaminated with germs or their toxins. Often these infections are spread by the faecal-oral route. Examples of food- or water-borne diseases:

  • listeria
  • typhoid
  • botulism.

Find out more about common food-borne illnesses on the Ministry for Primary Industries website.

Read more information about food- and water-borne illnesses.

Through contaminated environment

Some infectious diseases are not spread by contact with an infected person but by contact with an environmental source such as animals, insects or soil. Examples of diseases spread this way are:

  • hydatids (animals)
  • malaria (insect)
  • dengue (insect)
  • tetanus (soil)
  • legionellosis (compost/soil).

What you can do

Not all infectious diseases are spread in the same way, and some are more infectious than others.

Learn about simple actions you can take to reduce the risk of spreading a disease: Prevent the spread of infectious disease.

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