Health

~10 mins

Health is the complete state of physical, mental, and social well-being, not merely the absence of disease or infirmity. It encompasses how your body functions, how your mind processes thoughts and emotions, and how you relate to others and society. Health is dynamic, constantly changing based on genetics, environment, lifestyle choices, and access to healthcare.

1) Health exists because living organisms must maintain complex internal systems in balance whilst adapting to external challenges. Your body contains trillions of cells working together, each requiring specific conditions to function. Health represents successful coordination between organs, tissues, and systems. When this coordination breaks down, disease occurs. Health isn't binary — it exists on a spectrum from optimal wellness to severe illness. Understanding health helps you make informed decisions about diet, exercise, sleep, stress management, and medical care to maximise your quality and quantity of life.

Related: Health | WHO | Public Health

2) The human body is an integrated system of organs working together to maintain life. Your heart pumps blood through 100,000 kilometres of blood vessels, delivering oxygen and nutrients to every cell whilst removing waste. Your lungs extract oxygen from air and expel carbon dioxide. Your brain controls everything through electrical and chemical signals. Your digestive system breaks down food into usable components. Your immune system defends against threats. Your kidneys filter blood and regulate fluid balance. Each system depends on others — heart failure affects brain function, lung problems strain the heart, digestive issues impact immunity.

Related: Human Body | Organ Systems | Homeostasis

3) Nutrition provides the building blocks and energy your body needs to function, repair, and grow. Food contains macronutrients (carbohydrates for energy, proteins for structure and enzymes, fats for energy storage and hormone production) and micronutrients (vitamins and minerals for specific biochemical reactions). Your digestive system breaks food into molecules small enough to absorb. Carbohydrates become glucose for immediate energy. Proteins become amino acids for building tissues. Fats become fatty acids for long-term energy and cell membranes. Poor nutrition causes deficiency diseases, impairs immune function, and contributes to chronic conditions like diabetes and heart disease.

4) Exercise strengthens your cardiovascular system, muscles, bones, and mental health through controlled physical stress that triggers beneficial adaptations. When you exercise, your heart pumps faster, delivering more oxygen to working muscles. Your muscles develop microscopic tears that heal stronger. Your bones respond to impact by increasing density. Your brain releases endorphins (natural painkillers) and grows new neural connections. Regular exercise reduces risk of heart disease, stroke, diabetes, osteoporosis, depression, and many cancers. It improves sleep quality, cognitive function, and immune response. The key is consistency — even moderate activity like brisk walking provides substantial health benefits.

5) Sleep is when your body and brain perform essential maintenance that can't happen whilst awake. During sleep, your brain clears metabolic waste through the glymphatic system, consolidates memories by strengthening important neural pathways, and processes emotions. Your body releases growth hormone for tissue repair, your immune system produces infection-fighting cells, and your muscles recover from daily use. Sleep occurs in cycles — light sleep, deep sleep, and REM (rapid eye movement) sleep when most dreaming occurs. Adults need 7-9 hours nightly. Sleep deprivation impairs immune function, increases accident risk, contributes to obesity and diabetes, and affects mental health.

Related: Sleep | Sleep Cycles | Sleep Deprivation

6) Mental health encompasses your emotional, psychological, and social well-being, affecting how you think, feel, and behave. Your brain produces neurotransmitters (chemical messengers) like serotonin for mood regulation, dopamine for motivation and reward, and cortisol for stress response. Mental health exists on a spectrum from flourishing to severe mental illness. Common conditions include depression (persistent sadness and loss of interest), anxiety (excessive worry and fear), and bipolar disorder (mood swings between mania and depression). Mental health affects physical health — chronic stress weakens immunity, depression increases heart disease risk, and anxiety can cause digestive problems. Treatment includes therapy, medication, lifestyle changes, and social support.

7) The immune system is your body's defence network against harmful invaders like bacteria, viruses, fungi, and parasites. It has two main parts: innate immunity (immediate, general responses like inflammation and fever) and adaptive immunity (specific, learned responses that create immunological memory). White blood cells patrol your body — neutrophils attack bacteria, T-cells coordinate responses and kill infected cells, B-cells produce antibodies that neutralise specific threats. Vaccines work by training your adaptive immune system to recognise and respond quickly to dangerous pathogens. Autoimmune diseases occur when the immune system mistakenly attacks healthy tissue. Immunodeficiency makes you vulnerable to infections.

8) Disease occurs when normal body functions are disrupted by internal or external factors. Infectious diseases are caused by pathogens — bacteria (like tuberculosis), viruses (like influenza), fungi (like athlete's foot), or parasites (like malaria). Non-infectious diseases include genetic disorders (inherited DNA changes), autoimmune conditions (immune system attacking healthy tissue), cancers (uncontrolled cell growth), and degenerative diseases (progressive organ deterioration). Risk factors increase disease likelihood — genetics you can't change, but lifestyle factors like smoking, poor diet, lack of exercise, and excessive alcohol are modifiable. Prevention through vaccination, hygiene, healthy lifestyle, and regular screening is more effective and cheaper than treatment.

9) Cardiovascular disease affects your heart and blood vessels, causing more deaths globally than any other condition. Your heart is a muscular pump with four chambers that must beat rhythmically about 100,000 times daily. Coronary arteries supply blood to heart muscle — when these become blocked by fatty plaques (atherosclerosis), heart attacks occur. High blood pressure forces your heart to work harder, eventually weakening it. Risk factors include smoking (damages blood vessel walls), high cholesterol (contributes to plaque formation), diabetes (damages blood vessels), obesity, physical inactivity, and stress. Prevention involves regular exercise, healthy diet, not smoking, managing stress, and controlling blood pressure and cholesterol.

10) Cancer occurs when cells lose normal growth controls and divide uncontrollably, potentially spreading to other body parts. Normally, cells have built-in mechanisms that regulate division and trigger cell death when damage occurs. Cancer develops when genetic mutations disable these controls. Oncogenes (promote cell division) become overactive, whilst tumour suppressor genes (prevent excessive division) become inactive. Carcinogens like tobacco smoke, ultraviolet radiation, certain chemicals, and some viruses increase cancer risk by causing DNA damage. Cancer cells can invade nearby tissues and metastasise (spread) through blood or lymph systems. Early detection through screening improves treatment outcomes. Treatments include surgery, chemotherapy, radiation, immunotherapy, and targeted drugs.

Related: Cancer | Carcinogens | Metastasis

11) Diabetes is a group of metabolic disorders characterised by high blood glucose levels due to problems with insulin production or function. Insulin is a hormone produced by pancreatic beta cells that allows cells to absorb glucose from blood for energy. Type 1 diabetes occurs when the immune system destroys beta cells, requiring lifelong insulin injections. Type 2 diabetes develops when cells become resistant to insulin and the pancreas can't produce enough to overcome this resistance. High blood glucose damages blood vessels, nerves, kidneys, eyes, and other organs over time. Risk factors for Type 2 include obesity, physical inactivity, family history, and certain ethnicities. Management involves blood glucose monitoring, medication, diet control, and regular exercise.

Related: Diabetes | Insulin | Type 1 Diabetes

12) Infectious diseases spread through various transmission routes, and understanding these helps prevent outbreaks. Respiratory droplets carry diseases like influenza and COVID-19 when infected people cough, sneeze, or talk. Contaminated food and water spread diseases like cholera and salmonella. Vector-borne diseases like malaria are transmitted by mosquitoes, ticks, or other animals. Direct contact spreads diseases like skin infections. Sexual transmission affects diseases like HIV and syphilis. Some pathogens are highly contagious (spread easily), whilst others require specific conditions. Public health measures include vaccination programmes, sanitation systems, quarantine during outbreaks, contact tracing, and education about hygiene practices.

13) Vaccines are biological preparations that stimulate your immune system to develop immunity against specific diseases without causing the disease itself. They contain antigens — weakened or killed pathogens, parts of pathogens, or synthetic versions that resemble the pathogen. When introduced into your body, these antigens trigger an immune response, creating antibodies and memory cells. If you later encounter the actual pathogen, your immune system recognises and responds rapidly, preventing or reducing disease severity. Herd immunity occurs when enough people are vaccinated to protect the entire community, including those who can't be vaccinated due to medical conditions. Vaccines have eliminated smallpox globally and nearly eradicated polio.

Related: Vaccines | Herd Immunity | Immunisation

14) Antibiotics are medications that kill bacteria or prevent their growth, revolutionising medicine since penicillin's discovery in 1928. They work through various mechanisms — some disrupt bacterial cell walls, others interfere with protein synthesis or DNA replication. Antibiotics are specific to bacteria and don't work against viruses, fungi, or parasites. Antibiotic resistance develops when bacteria evolve mechanisms to survive antibiotic exposure. Overuse and misuse of antibiotics accelerates resistance development. Resistant bacteria can transfer resistance genes to other bacteria. Methicillin-resistant Staphylococcus aureus (MRSA) and tuberculosis are examples of dangerous resistant infections. Proper antibiotic use involves taking the complete prescribed course even if symptoms improve, and not demanding antibiotics for viral infections.

15) Hormones are chemical messengers produced by endocrine glands that regulate numerous body functions by travelling through your bloodstream to target organs. Insulin from the pancreas regulates blood sugar. Thyroid hormones control metabolism. Cortisol from adrenal glands manages stress response. Growth hormone from the pituitary promotes development. Sex hormones (testosterone, oestrogen, progesterone) control reproduction and secondary sexual characteristics. Hormones work in feedback loops — when levels get too high or low, the body adjusts production. Hormone imbalances cause various health problems: diabetes (insulin), hypothyroidism (thyroid hormones), Addison's disease (cortisol), and menstrual irregularities (sex hormones). Treatment often involves hormone replacement therapy.

16) Genetics determines many aspects of your health through DNA sequences inherited from your parents. Your genome contains about 20,000 genes that provide instructions for making proteins. Some diseases are caused by single gene mutations — sickle cell anaemia, cystic fibrosis, and Huntington's disease. Others involve multiple genes interacting with environmental factors — heart disease, diabetes, and most cancers. Genetic testing can identify disease risks, guide treatment decisions, and inform family planning. Gene therapy involves introducing healthy genes to treat genetic disorders. Personalised medicine uses genetic information to tailor treatments to individual patients. Genetic counselling helps people understand their genetic risks and make informed healthcare decisions.

17) Ageing is the gradual decline in physiological function over time, affecting all body systems. Cellular ageing involves DNA damage accumulation, telomere shortening (protective chromosome caps), and decreased cellular repair mechanisms. Your cardiovascular system becomes less efficient, bones lose density, muscles lose mass and strength, immune function declines, and cognitive processing may slow. However, ageing varies greatly between individuals based on genetics, lifestyle, and environmental factors. Healthy ageing involves maintaining physical activity, eating nutritious foods, staying socially connected, managing chronic conditions, and engaging in mentally stimulating activities. Research into ageing mechanisms may lead to interventions that extend healthy lifespan.

18) Stress is your body's response to perceived threats or challenges, triggering physiological changes that prepare you for "fight or flight." When you encounter stress, your hypothalamic-pituitary-adrenal (HPA) axis activates, releasing cortisol and adrenaline. These hormones increase heart rate, blood pressure, and blood glucose whilst suppressing non-essential functions like digestion and immunity. Acute stress can be beneficial, improving performance and focus. However, chronic stress keeps these systems constantly activated, leading to health problems including cardiovascular disease, digestive issues, weakened immunity, anxiety, and depression. Stress management techniques include exercise, meditation, deep breathing, social support, and time management.

19) Addiction is a complex brain disorder characterised by compulsive engagement in rewarding stimuli despite adverse consequences. Addictive substances (alcohol, nicotine, opioids, cocaine) and behaviours (gambling, gaming) activate your brain's reward system, releasing dopamine in the nucleus accumbens. Repeated exposure changes brain chemistry and structure, particularly in areas controlling decision-making, impulse control, and stress response. Tolerance develops, requiring increasing amounts for the same effect. Withdrawal symptoms occur when the substance is discontinued. Risk factors include genetics (accounting for 40-60% of addiction risk), mental health conditions, trauma, and social environment. Treatment involves detoxification, behavioural therapy, medication, and ongoing support.

20) Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, serving as a protective mechanism. Nociceptors (pain receptors) detect harmful stimuli and send signals through peripheral nerves to your spinal cord, which relays them to your brain for interpretation. Your brain processes pain in multiple areas, explaining why pain has both sensory (location, intensity) and emotional (unpleasantness, suffering) components. Acute pain warns of injury and motivates protective behaviour. Chronic pain persists beyond normal healing time and may involve sensitisation of pain pathways. Pain management includes medications (analgesics, anti-inflammatories), physical therapy, psychological interventions, and sometimes surgical procedures.

Related: Pain | Pain Receptors | Chronic Pain

21) Pregnancy involves the development of a foetus within a woman's uterus over approximately 40 weeks, divided into three trimesters. Fertilisation occurs when sperm meets egg in the fallopian tube, forming a zygote that develops into an embryo, then foetus. The placenta develops to provide nutrients and oxygen whilst removing waste. Hormonal changes support pregnancy but can cause symptoms like nausea, fatigue, and mood changes. Prenatal care involves regular medical check-ups, screening tests, and lifestyle modifications to ensure healthy development. Common complications include gestational diabetes, pre-eclampsia, and preterm labour. Proper nutrition, avoiding alcohol and smoking, taking folic acid, and managing existing health conditions are crucial for healthy pregnancy outcomes.

22) Sexual health encompasses physical, emotional, mental, and social well-being related to sexuality, requiring positive, respectful approaches to sexual relationships. Sexually transmitted infections (STIs) spread through sexual contact — bacterial STIs like chlamydia and gonorrhoea are curable with antibiotics, whilst viral STIs like herpes and HIV are manageable but not curable. Safe sex practices include condom use, regular STI testing, limiting sexual partners, and honest communication about sexual history. Contraception prevents unwanted pregnancy through various methods — hormonal (birth control pills), barrier (condoms), intrauterine devices (IUDs), and permanent sterilisation. Sexual dysfunction can affect desire, arousal, or orgasm, with causes including medical conditions, medications, psychological factors, and relationship issues.

Related: Sexual Health | STIs | Safe Sex

23) Environmental health examines how environmental factors affect human health, including air quality, water safety, chemical exposures, and climate change. Air pollution from vehicle emissions, industrial processes, and burning fossil fuels causes respiratory diseases, cardiovascular problems, and premature death. Water contamination with bacteria, viruses, chemicals, or heavy metals leads to gastrointestinal illness, neurological problems, and cancer. Occupational hazards expose workers to dangerous substances, noise, radiation, or repetitive motions. Climate change affects health through extreme weather events, changing disease patterns, food insecurity, and air quality degradation. Environmental health protection involves regulation of pollutants, monitoring of environmental quality, and promotion of sustainable practices.

24) Healthcare systems organise the delivery of health services to populations, varying significantly between countries in funding, structure, and outcomes. Universal healthcare systems provide coverage to all citizens, funded through taxes or mandatory insurance. Private healthcare systems rely on individual insurance or direct payment. Mixed systems combine public and private elements. Healthcare delivery involves primary care (first contact, ongoing care), secondary care (specialist services), and tertiary care (highly specialised treatment). Public health focuses on population-level prevention and health promotion. Healthcare quality depends on accessibility, affordability, safety, effectiveness, and patient satisfaction. Healthcare costs continue rising due to ageing populations, chronic disease prevalence, and expensive medical technologies.

25) Medical diagnosis involves identifying diseases or conditions based on signs (observable findings) and symptoms (patient-reported experiences). Doctors use medical history, physical examination, and diagnostic tests to gather information. Laboratory tests analyse blood, urine, or tissue samples for abnormal values. Imaging techniques like X-rays, CT scans, MRI, and ultrasound visualise internal structures. Biopsy involves removing tissue samples for microscopic examination. Differential diagnosis considers multiple possible conditions that could explain the patient's presentation. Evidence-based medicine uses research findings to guide diagnostic and treatment decisions. Diagnostic errors can lead to delayed treatment, unnecessary procedures, or inappropriate therapy, highlighting the importance of thorough evaluation and clinical reasoning.

26) Pharmacology studies how drugs interact with living systems to treat, prevent, or diagnose disease. Drugs have specific mechanisms of action — they may block receptors, inhibit enzymes, replace missing substances, or kill pathogens. Pharmacokinetics describes what the body does to drugs: absorption (entering bloodstream), distribution (reaching target tissues), metabolism (chemical modification), and excretion (elimination from body). Pharmacodynamics describes what drugs do to the body — dose-response relationships, therapeutic effects, and side effects. Drug interactions occur when multiple medications affect each other's action. Adverse drug reactions range from mild side effects to life-threatening complications. Personalised medicine considers genetic variations that affect drug response.

27) Surgery involves operative procedures to treat injuries, diseases, or deformities through manual or instrumental techniques. Surgical procedures range from minor outpatient operations to major procedures requiring hospitalisation. Anaesthesia prevents pain during surgery — local anaesthesia numbs specific areas, regional anaesthesia blocks sensation in body regions, and general anaesthesia renders patients unconscious. Minimally invasive surgery uses small incisions and specialised instruments, reducing recovery time and complications compared to open surgery. Robotic surgery provides enhanced precision and visualisation. Surgical risks include bleeding, infection, anaesthetic complications, and organ damage. Preoperative assessment evaluates surgical risks, whilst postoperative care monitors recovery and manages complications.

28) Rehabilitation medicine helps people recover function and independence after illness, injury, or surgery. Physical therapy uses exercise, manual techniques, and modalities to improve mobility, strength, and function. Occupational therapy helps people perform daily activities and return to work. Speech therapy addresses communication and swallowing disorders. Rehabilitation involves multidisciplinary teams including doctors, therapists, nurses, and social workers. Neurological rehabilitation helps patients with stroke, spinal cord injury, or traumatic brain injury regain lost abilities and learn compensatory strategies. Cardiac rehabilitation improves outcomes after heart attacks or heart surgery. Pulmonary rehabilitation benefits people with chronic lung diseases. Early intervention and intensive therapy often lead to better functional outcomes.

29) Preventive medicine focuses on preventing disease occurrence, progression, and complications rather than treating established conditions. Primary prevention prevents disease before it occurs through vaccination, healthy lifestyle promotion, and environmental modifications. Secondary prevention detects and treats preclinical disease through screening programmes like mammograms for breast cancer, colonoscopies for colorectal cancer, and blood pressure checks for hypertension. Tertiary prevention prevents complications in people with established disease through medication compliance, lifestyle modifications, and regular monitoring. Population-based prevention addresses social determinants of health like poverty, education, and environmental factors. Cost-effectiveness analyses show that prevention often provides better health outcomes per pound spent than treatment.

30) Global health addresses health issues that transcend national boundaries, requiring international cooperation and solutions. Health disparities exist between and within countries — life expectancy ranges from under 50 years in some African countries to over 85 years in Japan and Switzerland. Infectious diseases like tuberculosis, malaria, and HIV disproportionately affect low-income countries. Non-communicable diseases increasingly burden middle-income countries undergoing epidemiological transition. Global health initiatives include disease eradication programmes, vaccination campaigns, maternal and child health interventions, and health system strengthening. International organisations like WHO coordinate global health efforts. Climate change, migration, trade, and travel create new global health challenges requiring collaborative responses.

↩ Back to Encyclopedia