What is health, and what is public health?
The World Health Organization describes health as complete physical, mental and social well-being, not just the absence of disease. Public health is the science and art of preventing disease and promoting health for a whole population through the organised efforts of society.
Its three main jobs: prevent (vaccines, clean water, safe food), promote (teach healthy habits: diet, exercise, no tobacco) and protect (watch for outbreaks, respond to emergencies). Health also depends on social factors: income, education, housing, clean air and fair access to care. A responsible health citizen follows vaccination schedules, keeps surroundings clean and does not spread rumours.
How infectious diseases spread
Pathogens (bacteria, viruses, fungi, protozoa) cause infectious diseases. They spread by:
- Air: coughs and sneezes (flu, tuberculosis, measles).
- Water and food: cholera, typhoid.
- Direct contact: touch, body fluids.
- Vectors: insects like mosquitoes (malaria, dengue).
An epidemic is a sudden rise in cases in one region; a pandemic spreads across continents. History has many: the plague in the 1300s, cholera in the 1800s, influenza in 1918, COVID-19 from 2020. The R number is how many people one sick person infects on average: above 1 the outbreak grows, below 1 it shrinks.
Non-communicable diseases (diabetes, heart disease, cancer) do not spread between people; public health fights them by promoting healthy lifestyles.
Prevention, vaccination and herd immunity
Prevention is cheaper and kinder than cure. Key tools: hand washing with soap, safe drinking water, toilets and sewage treatment, food safety, mosquito control, masks and isolation of sick people, quarantine of exposed people.
A vaccine contains a harmless form or part of a germ. It trains the immune system to make antibodies and memory cells, so a real infection is stopped quickly. When a large share of people are immune, the germ cannot find enough new hosts, so even those who cannot be vaccinated (newborns, very sick people) are protected. This is herd immunity. For very contagious diseases like measles, about 95% coverage is needed.
Governments also use health promotion policies: tobacco taxes, school meals, clean-air rules, campaigns and free screening.
Health systems and surveillance
Health care is organised in levels:
- Primary: first contact, close to home: clinics, community health workers, vaccination, mother and child care.
- Secondary: district hospitals with doctors for common specialities.
- Tertiary: large specialist hospitals for complex cases (heart surgery, cancer).
Many countries see health care as a right and aim for universal health coverage, so that people do not fall into poverty because of medical bills. Surveillance means collecting and checking case data regularly; a sudden spike warns of an outbreak so teams can respond early.
History of care: from traditional healers, to hospitals, to the germ theory (1860s–80s), vaccines, antibiotics (penicillin, 1928) and today's national health programmes.
Key formulas and definitions
- Health = physical + mental + social well-being
- Chain of infection: pathogen → way out → spread → way in → new host
- R > 1: outbreak grows · R < 1: outbreak shrinks
- Herd immunity threshold ≈ 1 − 1/R₀ (e.g. R₀ = 4 → 75%)
- Levels of care: primary → secondary → tertiary
- Epidemic: one region · Pandemic: many countries
Worked examples
1. A disease has R₀ = 5. What share of people must be immune for herd immunity?
Step 1: threshold = 1 − 1/R₀. Step 2: 1 − 1/5 = 1 − 0.2 = 0.8. Step 3: about 80% of the population must be immune.
2. In a town, dengue cases jump from 5 to 60 in two weeks after heavy rain. What should the public health team do?
Surveillance spotted the spike. Dengue spreads through mosquitoes that breed in standing water. Actions: remove standing water (coolers, tyres, pots), spray and use nets, tell people to wear full sleeves, and prepare clinics to test and treat cases.
3. Why do we wash hands with soap for about 20 seconds rather than just rinse?
Soap molecules break the oily outer layer of many germs and lift dirt so water can wash it away. Rubbing for about 20 seconds gives soap time to work on all surfaces of the hands. A quick rinse leaves most germs behind.
Common mistakes
- Thinking health only means 'not sick'. Mental and social well-being are part of health.
- Believing vaccines only protect the person who gets them. High coverage also protects others through herd immunity.
- Mixing up isolation (keeping sick people apart) and quarantine (keeping exposed but not-yet-sick people apart).
- Thinking antibiotics cure viral diseases like flu or COVID-19. Antibiotics act on bacteria only.