Health and Disease

Defining and categorising health and disease is not as simple as it may sound. Many different definitions and categories exist, and in this article, we will explore some of these. Let’s get into it. 

Health and Disease Health and Disease

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Table of contents

    What is health?

    There are many different definitions of the term ‘health’. One definition describes health as an individual being “free from disease or an impairment”.

    However, issues arise with this definition. Firstly, some people may have a disease or abnormality but feel perfectly well and live normally. For example, many people have peptic ulcers and other infections but do not experience any problems. They may not even know they have the disease and do not seek treatment.

    Another issue with this definition is that while someone may be in perfect health today, they could be found to be ill tomorrow through a novel (new) medical diagnostic method. This inconsistency renders the definition problematic.

    A more appropriate definition of health is:

    Health: The state of complete physical, mental and social well-being.

    This definition considers the individual's physical health and the psychological and social aspects of an individual’s well-being.

    What is disease?

    Some may define the term ‘disease’ as the opposite of health, but a more accurate description of disease is as follows:

    Disease: A state of illness or sickness that produces various signs or symptoms.

    Disease is not necessarily the direct result of physical injury but rather a disorder of the body or brain’s biochemistry, physiology and/or anatomy.

    Diseases can be either acute or chronic. An acute disease is any disease with a rapid onset with fast-changing symptoms and is short-lived. It can typically be cured with treatment, such as medication or surgery. An example of acute disease is appendicitis.

    However, a chronic disease is any disease that lasts for an extended period and requires ongoing medical treatment. It cannot typically be cured; however, medication or other procedures can help to manage some of the symptoms. An example of a chronic disease is terminal cancer.

    Categories of disease

    Diseases can be classified into different categories, depending on the nature of the disease and how it affects the body. They can also be classified regarding the tissue they affect, their cause and other reasons. Each classification serves to help us understand a specific aspect of a disease and relate it to similar diseases according to said classification.

    Table 1. Example classification of diseases.






    Infection from a pathogen

    Infection resulting from a pathogen that may be transmitted from a source, e.g., animal




    Causes could be genetic, lifestyle or infectious.

    Any disease that causes a gradual decline of size or function of an organ over time. This is often observable in a patient, e.g., gradual mental decline for diseases affecting the brain.

    Alzheimer’s disease;



    Not receiving enough nutrients in your diet.

    Any nutritional disease caused by an inadequate diet results in starvation



    Genetic cause where specific genes are faulty.

    Parents may pass faulty genes to their children, causing the disease to develop in the child.

    Cystic fibrosis;

    sickle-cell anaemia


    Physical damage to the body.

    Any disease that causes permanent or temporary damage to any tissues, organs or other structures of the body


    autoimmune diseases;



    Changes in brain chemistry; mental or physical trauma

    Any disease affecting the mind.




    Diseases may alternatively be categorised simply into two categories; communicable diseases and non-communicable diseases.

    Communicable diseases can be passed on from one organism to another. They include all infectious diseases since infective pathogens can be passed on between organisms.

    Non-communicable diseases cannot be passed on from one organism to another, including every disease except infectious diseases.

    Interactions between different types of diseases

    Sometimes, communicable diseases can cause or increase the likelihood of developing a non-communicable disease.

    Take HIV, for example. HIV, which stands for Human Immunodeficiency Virus, is a virus that attacks the immune system, causing AIDS - a communicable disease. The virus infects and replicates inside immune cells in the blood. The virus particles eventually burst the cells, killing them in the process. Thus, the new virus particles can infect more immune cells and weaken the immune system quickly. This makes it easier for other diseases to develop in the patient; for example, AIDS patients are more likely to develop a type of skin cancer called Kaposi’s sarcoma, caused by the Epstein-Barr virus, or develop cancer (a non-communicable disease) because the immune system is unable to fight the growth of cells as it would if it were in healthy conditions.

    Health and disease statistics

    Now that we’ve covered the concepts of health and disease, let’s look at how statistics can provide meaningful information about health and disease. Learn more about health statistics in our article here.

    Heath statistics is the science of collecting, summarising and interpreting data on health and disease in populations.

    Everything we know about the patterns of health and disease in populations is due to statistics.

    Three indicators can provide scientists and health professionals with crucial information about how a disease is progressing in a population, these are:

    • Incidence – measures the number of new cases of the disease within a given period

    • Prevalence – measures the number of people with the disease in a population

    • Mortality – measures the number of people who have died from the disease

    The World Health Organisation (WHO) is an inter-governmental body that collects health statistics and data globally. While specific governments assess disease patterns within their own country, the WHO assesses disease patterns across all countries.

    A significant role of the WHO is to monitor infectious diseases on a global scale. They collect data from individual countries to plan and implement strategies to combat infectious diseases, such as providing free vaccinations to regions urgently requiring them.

    Data gathered by the WHO show significant geographical differences in the spread of infectious diseases. Infectious and parasitic diseases are highly prevalent in Africa and South Asia, while the rest of the world has comparatively lower infection levels. Read our Health Statistics article to learn more about why these differences exist.

    Health and Disease - Key takeaways

    • There are many definitions of health.

    • Health is the state of complete physical, mental and social well-being

    • Disease is a state of illness or sickness that produces specific signs or symptoms

    • Diseases can be classified according to different parameters, for example, "communicable vs non-communicable diseases."

    Frequently Asked Questions about Health and Disease

    What are the 4 types of diseases? 

    Types of diseases include: infectious diseases, deficiency diseases, inherited diseases, physical diseases, self-inflicted, degenerative diseases and mental diseases. 

     How do you define a disease? 

    Disease can be defined as a state of illness or sickness that produces specific signs or symptoms. 

    What is the difference between health and disease? 

    Health is the state of complete physical, mental and social well-being, while disease is the state of illness or sickness that produces specific signs or symptoms

    What are the 10 most common diseases? 

    Ischaemic heart disease, 
    Chronic Obstructive Pulmonary Disease (COPD) 
    Lower respiratory infections, 
    Neonatal disorders, 
    Trachea, bronchus and lung cancers
    Diabetes mellitus
    Alzheimer’s disease and other dementias
    Diarrheal diseases
    Tuberculosis (TB)
    Liver cirrhosis

    What causes disease? 

    Disease can have various causes, such as genetic causes or environmental causes or lifestyle causes.  

    Test your knowledge with multiple choice flashcards

    What is Health Statistics and what does it involve?

    What is the significance of Health Statistics in healthcare decision-making?

    What role does Health Statistics play during public health emergencies?

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