What Is Disease?
Every day, millions of people around the world are affected by disease. But what exactly is a disease? Is it just being sick, or is there more to it? Understanding what disease means is the first step to understanding how we fight it.
Before You Begin
Think about the last time you or someone you knew was unwell. They might have had a cold, an allergy, or something more serious.
Write down your answers before reading on:
- What do you think made them sick?
- Could someone else catch what they had?
- What is the difference between being sick and having a disease?
Know
- The definition of disease and disorder
- The difference between infectious and non-infectious disease
- Examples of each type of disease
Understand
- Why distinguishing disease type matters for treatment
- How diseases affect the body at different levels
- The relationship between symptoms and disease
Can Do
- Classify diseases as infectious or non-infectious
- Explain what disease means in scientific terms
- Give examples of diseases from each category
What Is Disease?
Moving beyond "feeling sick" to a scientific definition
Disease Types
In science, disease is not just "feeling unwell." It is a condition that impairs the normal functioning of the body or mind.
Diseases can be caused by many things:
- Pathogens (bacteria, viruses, fungi, protists)
- Genetic factors (inherited conditions)
- Lifestyle choices (diet, exercise, smoking)
- Environmental exposure (radiation, toxins, pollutants)
- Immune system malfunction (autoimmune diseases)
Importantly, disease is different from injury. A broken bone or a cut is damage, not disease. Disease involves some ongoing process that disrupts normal body function.
Infectious Disease
When pathogens attack and spread
Infectious diseases are caused by pathogens and can spread from one organism to another. They are also called communicable diseases.
Examples include:
- Common cold — caused by viruses (rhinoviruses)
- Influenza — caused by influenza viruses
- COVID-19 — caused by SARS-CoV-2 virus
- Tuberculosis — caused by Mycobacterium tuberculosis bacteria
- Malaria — caused by Plasmodium protists, spread by mosquitoes
Infectious diseases can spread through the air, water, direct contact, or via vectors (organisms that carry pathogens). This is why isolation, handwashing, and vaccination are so important.
Non-Infectious Disease
When the cause is inside or environmental
Non-infectious diseases are not caused by pathogens and cannot spread from person to person. They are also called non-communicable diseases (NCDs).
Major categories include:
- Lifestyle diseases: Cardiovascular disease, type 2 diabetes, obesity (linked to diet, exercise, smoking)
- Genetic diseases: Cystic fibrosis, Huntington's disease, sickle cell anaemia (inherited from parents)
- Environmental diseases: Asthma, some cancers (linked to pollution, radiation, toxins)
- Degenerative diseases: Alzheimer's, osteoarthritis (body tissues break down over time)
Non-infectious diseases are the leading cause of death worldwide. According to the World Health Organization, NCDs cause about 74% of all deaths globally.
Comparing Disease Types
Key differences that matter for treatment and prevention
| Feature | Infectious Disease | Non-Infectious Disease |
|---|---|---|
| Cause | Pathogen (bacteria, virus, etc.) | Genetics, lifestyle, environment |
| Spread | Can spread between organisms | Cannot spread |
| Examples | Flu, COVID-19, malaria | Diabetes, asthma, cancer |
| Prevention | Vaccination, hygiene, isolation | Healthy lifestyle, screening |
| Treatment | Antibiotics, antivirals | Lifestyle changes, medication, surgery |
Understanding whether a disease is infectious or non-infectious is crucial because it determines how we prevent and treat it. You cannot catch diabetes from someone, but you can catch influenza.
Common Misconceptions
"All diseases are caused by germs." No — many diseases are non-infectious. Cancer, diabetes, asthma, and genetic disorders are not caused by germs and cannot be caught from someone else.
"If you feel sick, you must have a disease." Not always. Feeling sick can be caused by temporary conditions like hunger, tiredness, or anxiety. Disease involves a specific disruption of normal body function.
Disease in Australia
Infectious diseases in Australia: While Australia has low rates of many infectious diseases compared to developing nations, outbreaks still occur. Ross River virus (transmitted by mosquitoes) affects thousands of Australians each year, mainly in rural and regional areas. Q fever, spread from livestock, is another significant concern for agricultural workers.
Non-infectious disease burden: Cardiovascular disease is Australia's biggest killer, responsible for about 1 in 4 deaths. Type 2 diabetes affects over 1.3 million Australians and is one of the fastest-growing chronic conditions. The Australian Institute of Health and Welfare tracks these trends to guide public health policy.
Closing the Gap: Aboriginal and Torres Strait Islander Australians experience higher rates of both infectious and non-infectious diseases than non-Indigenous Australians. The Closing the Gap initiative aims to reduce these disparities through targeted health programs and community-led interventions.
✍ Copy Into Your Books
▾Disease Definitions
- Disease = condition that impairs normal body function
- Infectious = caused by pathogen, can spread
- Non-infectious = not caused by pathogen, cannot spread
Examples
- Infectious: flu, COVID-19, malaria, tuberculosis
- Non-infectious: diabetes, asthma, cancer, heart disease
Key Difference
- Infectious spreads between organisms
- Non-infectious does not spread