The chain of infection is a model used to understand how infectious diseases are transmitted from one host to another.
Breaking any link in this chain can effectively prevent the spread of infection. It is important to understand the chain of infection in health and social care settings.
Below, we detail each component of the chain of infection:
1. Infectious Agent
The first link is the pathogen, or microorganism, that causes the disease. This can be a virus, bacterium, fungus, or parasite. The infectious agent must be present for the disease to occur.
Examples:
- Viruses: Influenza, HIV
- Bacteria: Tuberculosis, E. coli
- Fungi: Candida (which can cause thrush)
- Parasites: Plasmodium (which causes malaria)
2. Reservoir
The reservoir is the natural habitat where the infectious agent lives, grows, and multiplies. This can be a human, animal, or environmental source.
Examples:
- Humans: Carriers of diseases like hepatitis B
- Animals: Carriers of diseases like rabies (spread by mammals such as dogs and bats; in the UK it is only found in some bats) or avian flu (in birds)
- Environment: Contaminated water, soil, or surfaces can act as reservoirs for organisms like Legionella bacteria.
3. Portal of Exit
The portal of exit is the path by which the infectious agent leaves the reservoir. This is necessary for the organism to find a new host.
Examples:
- Respiratory tract: Coughing and sneezing can spread respiratory infections such as flu. See the NHS tuberculosis guide for how TB spreads.
- Gastrointestinal tract: Vomiting and diarrhoea spread illnesses like cholera or norovirus.
- Blood: Contact with infected blood, for example through a needlestick injury, can spread blood-borne viruses such as hepatitis B and HIV. Donated blood is tested, so infection from a transfusion is very unlikely (see the NHS blood transfusion guide).
4. Mode of Transmission
The mode of transmission is how the infectious agent is transferred from the reservoir to a new host. Transmission can be direct or indirect.
- Direct Transmission: Includes direct contact (touching, kissing) and spread of respiratory droplets over short distances (sneezing, coughing). The National Infection Prevention and Control Manual sets out the current contact, droplet and airborne routes.
- Indirect Transmission: Involves an intermediary:
- Airborne: Some infections spread through tiny respiratory particles (aerosols) that can stay in the air. Follow your organisation’s procedures and the National Infection Prevention and Control Manual.
- Vehicle-borne: Food, water, and objects like medical equipment can carry pathogens, e.g., Salmonella via contaminated food.
- Vector-borne: Insects like mosquitoes (malaria) or ticks (Lyme disease) serve as carriers.
5. Portal of Entry
The portal of entry is the route through which the infectious agent enters a new host. The portals of entry are often similar to the portals of exit.
Examples:
- Respiratory tract: Inhalation of airborne droplets.
- Gastrointestinal tract: Ingestion of contaminated food or water.
- Skin/mucous membranes: Open wounds, insect bites, or mucous membranes in the eyes, mouth, nose.
6. Susceptible Host
The final link is a susceptible host—someone who is not immune to the infectious agent. Various factors can affect susceptibility, including age, health status, vaccination status, and immune system function.
Examples:
- Newborns and older adults are often more susceptible to infections because of weaker immune systems.
- Individuals with chronic illnesses or those receiving immunosuppressive treatments are also at increased risk.
Breaking the Chain of Infection
To prevent the spread of diseases, interventions can be targeted at any point in this chain:
- Infectious Agent: Use anti-microbial agents and practice good hygiene to kill or reduce pathogens.
- Reservoir: Clean and disinfect sources of infection, and follow your organisation’s procedures on isolation and the National Infection Prevention and Control Manual.
- Portal of Exit: Use control measures such as covering coughs and sneezes, safe disposal of waste and PPE where a risk assessment says it is needed. Follow the National Infection Prevention and Control Manual.
- Mode of Transmission: Regular handwashing, safe food practices, wearing personal protective equipment (PPE), and using insect repellent can disrupt transmission.
- Portal of Entry: Use barriers such as gloves and protective clothing when a risk assessment shows they are needed. See the standard infection control precautions.
- Susceptible Host: Increase host resistance through vaccinations, proper nutrition, and medical treatment.
By understanding and interrupting the chain of infection, health professionals and individuals can reduce and control the spread of infectious diseases.