With the global COVID-19 pandemic having a severe impact on all aspects of society and the health of people worldwide, it is now more important than ever to update your knowledge on the spread and containment of infectious diseases, and what you can do to help break the chain of infection and contain the spread.
What is an Infectious Disease?
Infectious diseases are caused by pathogenic organisms including viruses, bacteria, fungi and parasites. They may be transmitted by other people, animals (zoonotic diseases) or environmental sources (e.g. water) depending on the specific pathogen (Mayo Clinic 2019).
Infectious diseases differ from non-infectious diseases (noncommunicable diseases) such as deficiency diseases, hereditary diseases and physiological diseases, because rather than developing on their own, they are caused by organisms that are not naturally found in the body (AIDSinfo n.d.).
The symptoms and severity of infectious diseases vary greatly depending on the pathogen causing them (Mayo Clinic 2019).
The Body’s Response to Infection
Types of Pathogen
Bacteria are small, singular cells that are able to survive in a variety of environments - sometimes in extreme cold or heat - without a living host. Bacteria exist in the body naturally and are mostly harmless, but others can cause infections. Bacterial infections can usually be treated with antibiotics (Mayo Clinic 2017).
Common bacterial infections include (but are not limited to):
Strep throat (group B strep);
Urinary tract infection; and
Viruses are even smaller than bacteria, and are not cells - instead they invade and multiply within the cells of a host. They can not survive without a living host (Mayo Clinic 2017; Healthdirect 2018).
Viral infections include (but are not limited to):
Ebola virus disease;
Hepatitis A, B and C;
Severe Acute Respiratory Syndrome (SARS); and
(Queensland Health n.d.)
Fungi are single-celled or multicellular organisms that reproduce using spores. They include molds, yeasts and mushrooms (Biology Dictionary 2017).
There are millions of fungi species, most of which are harmless. Only a few hundred varieties can cause illness (CDC 2019).
Fungal infections include (but are not limited to):
Fungal nail infections;
Candida infection (thrush) of the mouth, nouse, esophagus or vagina.
Parasites are organisms that live in or on another organism and feed on them. Their size varies, with some parasites large enough to be visible with the naked eye (Sepsis Alliance 2017).
Disease-causing parasites are separated into three main classes:
Protozoa are microscopic and single-celled. They are most commonly transmitted between humans through a fecal-oral route.
Helminths are parasitic worms that can generally be seen with the naked eye. They are also commonly transferred through a fecal-oral route.
Ectoparasites (e.g. mosquitoes, lice, fleas, ticks) are organisms that attach and burrow into the skin. They can cause disease by themselves, but some ectoparasites, such as mosquitoes, can be transmitters of other, more serious pathogens.
(CDC 2016; Ho 2019)
Parasitic infections include (but are not limited to):
Lice infestation (body, hair, head, public);
How are Infections Transmitted?
The series of events enabling infections to spread between organisms is known as the ‘chain of infection’. There are six steps or ‘links’ in the chain that continue to repeat, and breaking any of these links will allow the chain to be dismantled (RCNi 2016).
The Chain of Infection:
1. Infectious Agent
The pathogen that causes the infection (NFID 2016).
The habitat where the infectious agent is living and multiplying. This can be a human, animal, object or environmental source such as water or soil (RCNi 2016; NFID 2016).
3. Portal of Exit
The means through which the infectious agent escapes from the reservoir. This can include bodily excretions, open wounds, aerosols and unclean objects (RCNi 2016; NFID 2016).
4. Mode of transmission
The way in which the infectious agent spreads from one place to another (RCNi 2016). Different infections use different modes of transmission.
These modes include:
Droplet transmission - The infectious agent spreads through large respiratory droplets in the air, which are then breathed in by another person. These droplets are released when an infected person coughs, sneezes or talks, travelling a short distance (about one metre) before falling to the ground. They may also contaminate nearby surfaces. If an uninfected person makes contact with these contaminated surfaces and touches their eyes, nose or mouth, this may also result in transmission.
Airborne transmission - Similarly to droplet transmission, the infectious agent spreads through respiratory droplets from an infected person. However, these droplets are much smaller in size, meaning they can stay suspended in the air for a longer period of time and be dispersed by air currents. Consequently, the air of the room of someone who has an airborne infection may be contaminated.
Direct contact - The infectious agent is spread through direct contact with an infected person/animal or their bodily fluids. This can include:
Touching an infected person (skin-to-skin);
Coming into contact with the bodily fluids of an infected person through activities such as kissing, breastfeeding or sexual intercourse;
Bodily fluids from an infected person entering the bloodstream of an uninfected person (through needles or breaks in the skin); and
Being bitten or scratched by an infected animal, or touching their waste.
Indirect contact - An uninfected person makes physical contact with a contaminated surface. A common example of this is faecal-oral spread, where tiny amounts of faeces from an infected person’s hands contaminate another surface. An uninfected person makes contact with this surface and then touches their mouth, allowing the particles of faeces to enter their body.
Insect transmission - Parasites such as mosquitoes and ticks may be carriers (‘vectors’) for other disease-causing pathogens such as malaria.
Food transmission - Some infectious agents can contaminate food or water, resulting in infection when it is consumed.
(SA Health n.d., 2019; Ministry of Health NZ 2017; Mayo Clinic 2019; Government of Nunavut n.d.)
5. Portal of Entry
The means through which the infectious agent enters the new host. Depending on the mode of transmission this can occur through:
Being breathed in;
Entering open wounds or cuts;
Entering the respiratory tract;
Entering the mucous membranes; and
Entering through medical instruments placed into the body.
(RCNi 2016; NFID 2016)
6. Susceptible Host
An uninfected individual who is susceptible to developing an infection when the infectious agent enters their body. Depending on the individual and the invading pathogen, they may be able to fight off the infection (RCNi 2016). Those who are more susceptible include:
People who are physically weak (e.g. due to malnutrition or dehydration).
Preventing Infection Transmission (Breaking the Chain)
In order to manage and prevent the spread of infectious diseases, it is important to interrupt the chain of infection at any of the six links.
The specific preventative measures used will depend on the pathogen and its mode of transmission, however the following list offers some suggested strategies for breaking the chain of infection at each link.
(Note: Infectious agents may still be present even if preventative measures are taken. However, the more measures implemented, the easier it will be to interrupt links of the chain. Prevention is most effective when a combination of measures are used.)