Classification of Diseases
Senior 1 · Biology
Classification of Diseases
Lesson 2 of 5
25 min
A nurse at a rural health centre looks back over one week's register. She sees: malaria, a common cold, sickle-cell anaemia, kwashiorkor, ringworm, type 2 diabetes, osteoarthritis in an old farmer's knees, and tuberculosis. Some of these patients could pass their illness to their families; others could not, however close they lived. To plan her work (which families to visit, who needs a bed net, who needs advice on diet), she must first sort the diseases by their cause. Biologists classify diseases in exactly this way.
Definition
Pathogen: an organism or virus that causes disease, such as a bacterium, a virus, a protoctist, a fungus or a parasitic worm.
Infectious disease (communicable disease): a disease caused by a pathogen, which can pass from one person or animal to another. Example: malaria.
Non-infectious disease (non-communicable disease): a disease not caused by a pathogen, which cannot be caught from another person. Example: sickle-cell anaemia.
Definition
Deficiency disease: caused by a lack of a nutrient in the diet. Example: kwashiorkor (lack of protein).
Inherited (genetic) disease: caused by a faulty gene passed from parents to children. Example: sickle-cell anaemia.
Degenerative disease: caused by the gradual wearing out or breakdown of a tissue or organ, usually with age. Example: osteoarthritis.
Lifestyle disease: caused mainly by the way a person lives: diet, physical activity, smoking or alcohol. Example: type 2 diabetes.
Figure 1: Diseases are first classified by whether a pathogen causes them. Infectious diseases are grouped by the kind of pathogen; non-infectious diseases by their main cause.
In Unit 2 you met the five kingdoms and the viruses; the pathogens that cause infectious diseases come from four of those kingdoms, plus the viruses. You will study how they spread in the next lesson.
| Type of non-infectious disease | Cause | Examples | Prevention or management |
|---|---|---|---|
| Deficiency | too little of a nutrient in the diet | kwashiorkor (protein), rickets (vitamin D and calcium), scurvy (vitamin C), iron-deficiency anaemia | a balanced diet (Unit 7) |
| Inherited | a faulty gene inherited from the parents | sickle-cell anaemia, haemophilia | cannot be prevented by the person; managed with medical care; genetic counselling helps couples |
| Degenerative | tissues wearing out, mostly with age | osteoarthritis, cataract, Alzheimer's disease | healthy habits slow them; some can be treated (cataract surgery) |
| Lifestyle | diet, lack of exercise, smoking, alcohol | type 2 diabetes, high blood pressure, lung cancer, liver cirrhosis | healthy diet, exercise, no smoking, little or no alcohol |
Worked example 1: classifying a disease step by step
Classify sickle-cell anaemia, using a series of questions.
Answer: sickle-cell anaemia is a non-infectious, inherited disease. The same questions, in the same order, work for any disease: pathogen? nutrient? genes? wear with age? habits?
Worked example 2: when a disease fits more than one group
The register records 40 patients in one week (illustrative figures): 18 malaria, 6 colds, 4 tuberculosis, 3 ringworm, 3 type 2 diabetes, 2 kwashiorkor, 2 osteoarthritis, 2 sickle-cell anaemia. (a) What percentage of cases were infectious? (b) Why might the nurse hesitate over the diabetes cases?
Warning
Common mistakes. (1) "All diseases can be caught." Only infectious diseases spread; you cannot catch sickle-cell anaemia, diabetes or kwashiorkor, so there is no reason to avoid people who have them. (2) "Non-communicable means not serious." Heart disease, cancer and diabetes can be very serious. (3) Mixing up the two kinds of anaemia: iron-deficiency anaemia is a deficiency disease, while sickle-cell anaemia is inherited. (4) "HIV is a lifestyle disease." HIV is caused by a virus, so it is infectious, even though behaviour affects the risk of catching it.
Communicable and Non-Communicable Disease (Freesciencelessons). Watch for how the two groups are defined and the examples of risk factors for non-communicable diseases.
Tip
Memory hook: "Can I catch it?" Yes means infectious; no means non-infectious. Then ask D-I-D-L: Deficiency (missing nutrient), Inherited (genes), Degenerative (wearing out), Lifestyle (habits).
Practice: classifying diseases
Key points
Sources and credits
Written by Ganzaa: the register scenario, Figure 1, the table, the worked examples (illustrative figures) and the practice questions.
