Topic 1.2 Understanding Microorganisms and Infectious Diseases

A sound understanding of microorganisms and infectious diseases provides the scientific foundation for effective infection prevention and control. Every day, humans, animals and the environment are exposed to millions of microorganisms. The overwhelming majority are harmless, and many are essential for maintaining healthy ecosystems and supporting life. Only a relatively small proportion of microorganisms have the potential to cause disease.

For Occupational Therapists (OTs) working with assistance animals, understanding microorganisms extends beyond simply recognising pathogens. It involves appreciating how microorganisms interact with humans, animals and the environment, recognising the conditions under which disease occurs, and applying this knowledge to minimise unnecessary risks while supporting safe participation in everyday occupations.

An evidence-informed understanding of microbiology also helps practitioners distinguish between genuine infection risks and common misconceptions surrounding animals in public spaces. Assistance animals are sometimes incorrectly perceived as inherently unhygienic or significant sources of disease. In reality, healthy assistance animals that receive appropriate veterinary care present very low infection risks when appropriate infection prevention measures are followed. Understanding the science behind infectious diseases enables Occupational Therapists to advocate confidently for clients while implementing proportionate, evidence-based infection prevention strategies.

This topic introduces the major groups of microorganisms, explains the difference between colonisation and infection, explores the concepts of infectious disease and immunity, and examines the factors that influence whether exposure to microorganisms results in illness.

What Are Microorganisms?

Microorganisms, often referred to as microbes, are microscopic living organisms or biological agents that are too small to be seen with the naked eye. They exist naturally throughout the environment and can be found in soil, water, air, food, plants, animals and humans.

Microorganisms play essential roles in maintaining healthy ecosystems. They assist with nutrient cycling, decomposition, digestion, immune system development and protection against disease. Without microorganisms, life on Earth would not be possible.

While some microorganisms are capable of causing disease, many are beneficial or harmless. Infection prevention therefore focuses on reducing exposure to harmful microorganisms (pathogens) rather than eliminating all microbial life.

The major groups of microorganisms encountered in healthcare and assistance animal practice include:

• bacteria
• viruses
• fungi
• parasites
• prions.

Each group differs in its structure, method of reproduction, transmission and response to treatment.

Bacteria

Bacteria are single-celled living organisms found almost everywhere in the natural environment. Many bacteria are beneficial and form part of the normal microbiome of humans and animals. They assist with digestion, produce essential vitamins and help prevent harmful microorganisms from establishing themselves.

Only a small proportion of bacteria are pathogenic and capable of causing disease.

Examples of bacterial infections include:

• Staphylococcus aureus
• Salmonella species
• Campylobacter species
• Escherichia coli (certain strains)
• Mycobacterium tuberculosis.

Bacterial infections may often be treated with antibiotics, although increasing antimicrobial resistance presents a growing global public health challenge.

Viruses

Viruses differ from bacteria because they are not considered living organisms in the traditional sense. Unlike bacteria, viruses cannot reproduce independently and require living host cells in order to replicate.

Once inside a host cell, viruses hijack the cell’s machinery to produce more viral particles, which can then spread to infect other cells or individuals.

Common viral infections include:

• influenza
• COVID-19
• respiratory syncytial virus (RSV)
• norovirus
• herpes simplex virus.

Antibiotics are ineffective against viral infections. Instead, prevention through vaccination, hygiene practices and infection prevention measures plays a particularly important role.

Fungi

Fungi include yeasts and moulds. Most fungi are harmless and are naturally present in the environment. Some form part of the normal microbial flora of healthy humans and animals.

Under certain circumstances, however, fungi may cause infection, particularly in individuals with weakened immune systems.

Examples include:

• ringworm (dermatophytosis)
• candidiasis (thrush)
• aspergillosis.

Some fungal infections are zoonotic and may occasionally be encountered in assistance animal practice, although appropriate veterinary care substantially reduces this risk.

Parasites

Parasites are organisms that live on or within another organism (the host) and obtain nutrients at the host’s expense.

Parasites may be microscopic or visible to the naked eye and include:

• protozoa
• worms (helminths)
• fleas
• ticks
• mites.

Many parasites have complex life cycles involving multiple hosts or environmental stages.

Routine parasite prevention programs, regular veterinary examinations and good hygiene significantly reduce the likelihood of parasite transmission from assistance animals.

Prions

Prions are abnormal proteins capable of causing progressive neurological disease by inducing normal proteins to become misfolded.

Unlike bacteria or viruses, prions contain no genetic material.

Prion diseases are extremely rare but include conditions such as:

• Creutzfeldt–Jakob disease (CJD)
• bovine spongiform encephalopathy (“mad cow disease”).

Prions are not considered a routine concern within occupational therapy or assistance animal practice but are included for completeness when discussing infectious agents.

Pathogens

A pathogen is any microorganism or biological agent capable of causing disease.

Whether exposure to a pathogen results in illness depends upon several interacting factors, including:

• the number of organisms present
• the route of exposure
• the virulence of the organism
• the immune status of the individual
• existing health conditions
• environmental factors
• availability of preventive measures such as vaccination.

Exposure alone does not necessarily result in infection or disease.

Colonisation, Infection and Disease

These three terms are often used interchangeably but have distinct meanings.

Colonisation

Colonisation occurs when microorganisms are present on or within the body without causing illness or tissue damage.

Humans and animals are naturally colonised by billions of microorganisms that form the normal microbiome.

For example, healthy dogs naturally carry numerous bacterial species on their skin and within their gastrointestinal tract without becoming ill.

Colonisation is both normal and beneficial.

Infection

Infection occurs when microorganisms enter the body, begin multiplying and trigger a response from the immune system.

An individual may have an infection without showing obvious symptoms.

Some infections remain localised, while others spread throughout the body.

Disease

Disease develops when infection results in tissue damage, impaired body function or clinical signs and symptoms.

Examples include:

• fever
• coughing
• diarrhoea
• vomiting
• inflammation
• pain
• fatigue.

Not every infection progresses to disease, and not every exposure leads to infection.

The Human and Animal Microbiome

Both humans and assistance animals possess complex communities of microorganisms collectively known as the microbiome.

The microbiome contributes to:

• digestion
• immune system development
• vitamin production
• protection against pathogens
• maintenance of healthy skin and mucous membranes.

Disruption of the microbiome through illness or inappropriate antibiotic use may increase susceptibility to infection.

Understanding the microbiome reinforces that microorganisms are not inherently harmful and that infection prevention seeks to maintain healthy microbial balance while preventing disease.

Host Factors Influencing Infection

Whether disease develops following exposure depends upon characteristics of both the microorganism and the host.

Host factors include:

• age
• pregnancy
• immune function
• chronic disease
• medications
• nutritional status
• stress
• vaccination status
• skin integrity.

Certain client populations commonly seen by Occupational Therapists—including older adults, infants, individuals undergoing chemotherapy, transplant recipients and people living with complex disabilities—may be more susceptible to infection and therefore require additional infection prevention measures.

Implications for Occupational Therapy Practice

Occupational Therapists do not diagnose infectious diseases; however, they play an important role in recognising potential infection risks, applying evidence-based precautions and promoting safe participation.

Understanding microorganisms enables practitioners to:

• conduct informed infection risk assessments
• educate clients and families using accurate scientific information
• challenge misconceptions regarding assistance animals
• collaborate effectively with healthcare and veterinary professionals
• select appropriate infection prevention strategies based on actual risk rather than fear
• support inclusive practice while protecting public health.

Scientific literacy also assists Occupational Therapists to interpret emerging research and adapt their practice in response to new infectious diseases and evolving public health guidance.

Key Practice Principle

Occupational Therapists working with assistance animals should understand the basic characteristics of microorganisms, the difference between colonisation, infection and disease, and the factors that influence infection risk. This knowledge enables practitioners to apply proportionate, evidence-informed infection prevention strategies that protect clients, assistance animals and the wider community while avoiding unnecessary restrictions based on misconceptions about animals or infectious disease.