Unit 10 Classification and patterns of disease
Key unit competence: Analyse social factors affecting good health at home, school, district, national and international level using knowledge of disease patterns.
Key ideas
- A disease is any condition that disrupts the normal functioning of the body or mind. Signs are what others can observe or measure (fever, rash); symptoms are what the patient feels (pain, nausea).
- Germ theory: many diseases are caused by microorganisms (pathogens) that enter and multiply in the body. Van Leeuwenhoek first saw microorganisms; Semmelweis (hand washing) and Lister (antiseptic surgery) reduced infections; John Snow traced cholera to a contaminated water pump; Pasteur disproved spontaneous generation and developed pasteurisation and vaccines; Robert Koch linked specific microbes to specific diseases (anthrax, TB).
- Koch's postulates: (1) the microbe is present in every diseased individual and not in healthy ones; (2) it can be isolated and grown in pure culture; (3) when the culture is put into a healthy susceptible host, it causes the same disease; (4) the same microbe is re-isolated from that host.
- Infectious (communicable) diseases are caused by pathogens: bacteria (cholera, typhoid, TB), viruses (HIV, influenza, Ebola, COVID-19, measles), fungi (ringworm), protoctists (malaria, amoebic dysentery, sleeping sickness) and worms (bilharzia). Non-infectious diseases: inherited (sickle cell, haemophilia), deficiency (scurvy, rickets, kwashiorkor), degenerative (osteoarthritis, Alzheimer's), mental (depression, schizophrenia), lifestyle/self-inflicted (lung cancer from smoking, cirrhosis from alcohol, obesity), and environmental (asbestosis).
- Patterns of disease: endemic (always present at a fairly steady level in a region, e.g. malaria in parts of Africa), epidemic (a sudden increase in cases far above normal in a community or region, e.g. a cholera outbreak), pandemic (an epidemic spreading across many countries and continents, e.g. COVID-19, HIV), sporadic (occasional isolated cases). Epidemiology studies the distribution and causes of disease using records of incidence (new cases) and prevalence (total cases).
- Typhoid: Salmonella typhi; contaminated food/water and flies; fever rising over a week, headache, rash, diarrhoea; treated with antibiotics; prevented by sewage treatment, safe water, food hygiene, vaccination. Cholera: Vibrio cholerae (comma-shaped); faecal-contaminated water; profuse "rice-water" diarrhoea and dehydration; treated with ORS and antibiotics. Malaria: Plasmodium spread by female Anopheles; fever and chills in cycles; nets, insecticides, draining breeding sites, antimalarials. TB: Mycobacterium tuberculosis by droplets; long cough, weight loss, night sweats; BCG vaccine, long course of several antibiotics (DOTS). HIV/AIDS and Ebola: body fluids.
- Mental disorders: depression (persistent sadness, loss of interest, sleep and appetite changes, hopelessness), anxiety disorders, schizophrenia (hallucinations, delusions), post-traumatic stress disorder (flashbacks after trauma), eating disorders. They are treatable with counselling, medication and social support; stigma prevents people seeking help.
- Early smoking and drug abuse (tobacco, alcohol, cannabis) harm the developing brain and lungs, lead to addiction, poor school performance, mental illness, risky behaviour, accidents and crime.
- Public health services in Rwanda: Ministry of Health and RBC (Rwanda Biomedical Centre) for disease surveillance, vaccination and outbreak response; referral, district and provincial hospitals; health centres and health posts; community health workers in every village; community-based health insurance (Mutuelle de Santé); health education, sanitation, safe water and nutrition programmes.
Common misconceptions
- "Germs only cause disease when the weather is cold." No: disease depends on exposure to pathogens and host immunity, not simply on weather (although seasons can affect vectors and spread).
- "An epidemic means a disease that is always present." No: always present is endemic; an epidemic is a sudden rise above normal.
- "Mental illness is caused by weakness of character." No: it has biological, psychological and social causes and can be treated.
- "Non-infectious diseases are not serious." No: heart disease, cancers and diabetes cause many deaths.
Worked examples
Example 1. A district records about 50 malaria cases every month all year, but cholera rises from 0 to 300 cases in two weeks after floods. Classify each pattern and explain.
Solution
Malaria is endemic: always present at a steady level because Anopheles mosquitoes and Plasmodium remain in the area. Cholera is an epidemic: a sudden rise far above normal, caused by floods contaminating water with faeces. If it spread to many countries, it would be a pandemic.
Example 2. Explain how Koch's postulates could be used to show that a bacterium causes a disease in goats.
Solution
Find the bacterium in all sick goats and not in healthy ones; isolate it and grow it in pure culture; inject the culture into healthy goats and check they develop the same disease; isolate the bacterium again from them and show it is identical.
Practice questions
State the germ theory and name three scientists who contributed to it.
Show answer
Many diseases are caused by microorganisms that enter and multiply in the body. Contributors: Pasteur, Koch, Lister, Semmelweis, John Snow, van Leeuwenhoek.
Common mistake: Saying germs arise spontaneously from decaying matter.
List Koch's postulates.
Show answer
Microbe present in all diseased hosts and not in healthy ones; isolated and grown in pure culture; causes the same disease in a healthy host; re-isolated and identified as the same microbe.
Common mistake: Leaving out the re-isolation step.
Distinguish between endemic, epidemic and pandemic diseases with examples.
Show answer
Endemic: always present at a steady level in an area (malaria). Epidemic: sudden large increase in cases in an area (cholera outbreak). Pandemic: epidemic over many countries/continents (COVID-19, HIV).
Common mistake: Mixing endemic and epidemic.
Give the cause, transmission and two prevention methods of typhoid.
Show answer
Cause: Salmonella typhi. Transmission: food and water contaminated with faeces, flies. Prevention: treat sewage, safe drinking water, food hygiene, hand washing, vaccination.
Common mistake: Saying typhoid is spread by mosquitoes.
Name the six groups of non-infectious diseases with an example of each.
Show answer
Inherited (sickle cell); deficiency (scurvy); degenerative (osteoarthritis); mental (schizophrenia); lifestyle/self-inflicted (lung cancer); environmental (asbestosis); eating disorders (anorexia).
Common mistake: Including malaria.
State four signs of depression and two ways to help.
Show answer
Persistent sadness; loss of interest; sleep and appetite changes; tiredness; feelings of worthlessness; thoughts of death. Help: listen and support, encourage professional help (counselling, health centre), reduce stigma.
Common mistake: Telling the person to just cheer up.
State four effects of early smoking and drug abuse.
Show answer
Addiction; damage to lungs and brain; poor school performance; mental illness; risky behaviour and accidents; crime and poverty.
Common mistake: Listing only health effects.
Describe the structure of public health services in Rwanda.
Show answer
Ministry of Health and RBC; referral hospitals; provincial and district hospitals; health centres and health posts; community health workers in villages; financed by Mutuelle de Santé and government; surveillance, vaccination and education programmes.
Common mistake: Naming only hospitals.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. An epidemic that spreads over many countries is a: a. pandemic 2. A disease always present at a steady level in an area is: b. schizophrenia 3. Hallucinations and delusions are signs of: c. endemic 4. Pasteurisation of milk works by: d. heating it to kill most pathogens Show answer
1 → a (pandemic) · 2 → c (endemic) · 3 → b (schizophrenia) · 4 → d (heating it to kill most pathogens)
- Example: COVID-19.
- Example: malaria in many parts of Africa.
- It is a serious mental disorder.
- Pasteur developed the method.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
inherited disease · oral rehydration solution · Mycobacterium tuberculosis · Salmonella typhi- The main treatment that saves lives in cholera is .
- Typhoid is caused by .
- Sickle cell anaemia is an example of a .
Show answer
1: oral rehydration solution · 2: Salmonella typhi · 3: inherited disease
- It replaces lost water and salts.
- It spreads in food and water.
- It is caused by a gene mutation.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Lung cancer caused by smoking is a: a. germ theory 2. The number of new cases of a disease in a period is its: b. lifestyle (self-inflicted) disease 3. Rickets is an example of a: c. incidence 4. The theory that many diseases are caused by microorganisms is the: d. deficiency disease Show answer
1 → b (lifestyle (self-inflicted) disease) · 2 → c (incidence) · 3 → d (deficiency disease) · 4 → a (germ theory)
- It is linked to behaviour.
- Prevalence is the total number of cases.
- It results from lack of vitamin D or calcium.
- Pasteur and Koch provided key evidence.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
droplets in the air · a type of referral hospital · community-based health insurance · Ignaz Semmelweis- The scientist who showed that hand washing reduced deaths in maternity wards was .
- Mutuelle de Santé is .
- Tuberculosis is spread mainly by .
Show answer
1: Ignaz Semmelweis · 2: community-based health insurance · 3: droplets in the air
- Doctors carried germs from autopsies.
- Members pay a yearly contribution.
- It is caused by Mycobacterium tuberculosis.