The Chain of Infection
A series of events has to happen to enable germs (when we speak of germs in this section, this refers to bacteria, fungi and viruses) to cause infections in a person. We call this the ‘Chain of Infection’. We can think of each part of the process as a separate ‘link’ in the chain. And if we can break a link at any part of the chain, we can stop infection arising.
By ‘reservoir’, we mean a place where germs can live and multiply. The ‘place’ can be a person – a patient/client or a member of staff – but it can also be any part of the surrounding area of a health care setting, furnishings in the patient’s/client’s room and the equipment we use in health care.
The portal of exit from the reservoir
The ‘portal of exit’ is the means by which the germ can escape from the reservoir. For instance, think about some germs (the infectious agent) sitting on top of a used commode (the reservoir). A health care worker comes along and touches the commode, and some of the germs move onto her hands. The health care worker’s hands are now the ‘portal of exit’ – the means by which the germs are able to move from the commode to another place.Other ‘portals’ can be people’s normal excretions (stools, vomit), body fluids (blood, saliva) and the air they breathe from their lungs, especially when they cough. The portal can vary from one infection to another (for example diarrhoeal infections are usually passed on via the patient's faeces).Germs can even be spread around on the tiny flecks of skin that peel off our bodies throughout the day and which form part of the dust that settles on all kinds of surfaces.Non-human portals of exit for germs include items of equipment that haven’t been properly cleaned, such as commodes, bed mattresses, pillows and reusable equipment.
The mode of transmission
This is how the germs move, or spread, from one place to another.This can happen in a number of ways, such as health care workers’ hands touching dirty equipment or contaminated medical instruments, or through the air (coughs, sneezes).
The portal of entry into the ‘host’
This means that the germs that have been moved from the reservoir now invade the person (the ‘host’). They can do this by entering wounds and cuts, being swallowed and being breathed in.
Patients who are having treatments that involve cutting the skin or placing medical instruments inside the body, such as a catheter being placed into the bladder or a feeding tube being passed down the throat, are also at risk of infection. Another example is people who inject drugs with used needles.
The susceptible host
Healthy people have their own defences which help them fight infection. This means that even if some harmful germs enter the body, the person can ‘fight them off’ and stay well. The ability of the body to defend itself against infection is called ‘immunity’.Some people, however, can’t fight infection effectively.
These include very young children, older people, people who are ill or who are receiving particular medicines that reduce their immunity, people with long-term health conditions like diabetes and those who are physically weak due to, for instance, malnutrition or dehydration.
People such as these are ‘susceptible hosts’ – meaning they are vulnerable to developing infection when their bodies are invaded by germs.
The infectious agent
The infectious agent is simply the germ that causes the infection.Germs are all around us and within us, and many play very important roles in keeping us healthy.The problem comes when a germ leaves its normal place to go elsewhere in the body – the germs that sit on your skin and which usually cause no harm, for instance, getting into a cut. The germ could then cause infection.There are also many germs that are not helpful to health and which cause disease. Entry of any of these germs into the body is likely to cause problems.