By: Fizza Zehra
Pathogenic organisms including viruses, bacteria, fungi and parasites cause infectious diseases. Other people, animals (zoonotic diseases), or environmental sources such as water depending on the specific pathogen (Mayo Clinic 2019), may transmit them.
Infectious diseases differ from non-infectious 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 (AIDS info n.d.).
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).
1. Infectious Agent
The pathogen that causes the infection (NFID 2016).
2. Reservoir
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 meter) 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, meaning they can stay suspended in the air for a longer period and be dispersed by air currents. Consequently, the air of the room with 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 faucal-oral spread, where tiny amounts of feces 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 feces 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.)
Infections may be indirectly transmitted if an uninfected person makes physical contact with a contaminated surface.
5. Portal of Entry
This is the means through which the infectious agent enters the new host. Depending on the mode of transmission, this can occur through:
Being breathed in;
Being swallowed;
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:
Very young children;
Older adults;
Immunocompromised people;
People who are ill;
People with a long-term health condition; and
People who are physically weak (e.g. due to malnutrition or dehydration).
(RCNi 2016)
The way to stop germs from spreading is by interrupting this chain at any link. Break the chain by cleaning your hands frequently, staying up to date on your vaccines (including the flu shot), covering coughs and sneezes and staying home when sick, following the rules for standard and contact isolation, using personal protective equipment the right way, cleaning and disinfecting the environment, sterilizing medical instruments and equipment, following safe injection practices, and using antibiotics wisely to prevent antibiotic resistance.
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