Disease interactions, cancer and impacts
Organisation • Animal tissues, organs and systems
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Key concepts
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Relationship between health and disease
Health refers to the normal functioning of systems and stable mental state. Disease refers to a condition that impairs normal function and reduces wellbeing. Cause → effect: exposure to pathogens, harmful chemicals, poor diet or genetic mutations causes disease, and disease reduces organ, tissue or cellular function. Limiting factors: presence of multiple risk factors increases likelihood of disease; age and genetic background alter risk and severity.
Interactions between different types of disease
Immune system defects reduce the number or effectiveness of defence cells (for example low lymphocyte counts), causing increased susceptibility to otherwise controllable infections; co-infections can worsen symptoms and slow recovery. Cause → effect: immune defect → reduced pathogen clearance → recurrent or severe infections. Limiting factors: access to treatment, nutrition and general health modify the degree of increased susceptibility.
Immune reactions and allergies
Allergy results from an exaggerated immune response to normally harmless substances (allergens). Cause → effect: exposure to an antigen or pathogen provokes antibody production or sensitised lymphocytes; subsequent exposure triggers rapid immune activation, inflammation and allergy symptoms such as sneezing, rash or asthma. Prior infection or immune activation can alter immune recognition and trigger or worsen allergic responses. Limiting factors: genetic predisposition and timing of exposure influence whether immune reactions become allergic.
Viruses and cancer
Some viruses infect host cells and integrate genetic material or produce proteins that disrupt normal cell-cycle control. Cause → effect: viral genes or chronic infection cause mutations or alter regulation of growth genes, producing uncontrolled cell division and tumour formation. Examples include human papillomavirus (HPV) and hepatitis viruses, which are associated with cervical and liver cancers respectively. Limiting factors: immune surveillance and vaccination reduce the cancer risk from viral infection.
Cancer as uncontrolled cell growth and division
Cancer arises when mutations or regulatory failures change cell behaviour so that mitosis occurs without normal growth controls. Cause → effect: mutation in genes that regulate the cell cycle (proto-oncogenes, tumour suppressors) → loss of controlled division → tumour formation and possible metastasis. Mutations accumulate with age and after exposure to mutagens. Limiting factors: DNA repair mechanisms and immune detection reduce progression; persistent exposure to mutagens increases risk.
Carcinogens and risk factors
Carcinogens include chemical agents and forms of ionising radiation that cause DNA mutations or promote uncontrolled cell growth. Cause → effect: exposure to carcinogens such as tobacco tar, asbestos, ultraviolet radiation or medical X-rays increases mutation rate and cancer risk. Lifestyle factors (smoking, alcohol, obesity) and some infections also increase risk. Limiting factors: dose, duration of exposure and individual DNA-repair capacity determine overall risk.
Benign versus malignant tumours
Benign tumours remain contained and usually stay within a membrane or local tissue; growth is often slower and removal cures the condition. Malignant tumours invade neighbouring tissues and can spread via the bloodstream or lymphatics (metastasis), producing secondary tumours. Cause → effect: loss of local growth control and acquisition of invasive properties leads to malignancy and systemic disease. Limiting factors: tumour type and stage at detection determine prognosis and treatment options.
Severe physical ill health and mental illness
Severe or chronic physical illness alters brain chemistry, lifestyle and social circumstances and therefore increases risk of depression and other mental illnesses. Cause → effect: chronic pain, reduced mobility or biochemical changes (for example due to alcohol damage) impair neurotransmitter balance and social functioning, producing anxiety or depression. Limiting factors: social support, access to mental-health care and rehabilitation modify the mental-health outcome.
Human and financial cost of non-communicable diseases
Non-communicable diseases (NCDs) such as cancer, heart disease and diabetes produce long-term care needs, disability and premature death. Cause → effect: increased prevalence of NCDs raises healthcare spending, reduces workforce productivity and imposes care costs on families and communities. National budgets face higher treatment and social-care costs; global economies face productivity losses and unequal impacts in low-resource regions. Limiting factors: prevention, screening programs and public-health policy reduce long-term costs.
Key notes
Important points to keep in mind