HUMAN HEALTH AND DISEASE

Health includes physical, mental and social well-being. This chapter covers infectious diseases, body defences, AIDS, cancer and the harmful consequences of drug and alcohol abuse.

Good health depends on balanced diet, hygiene, exercise, mental peace and a healthy social environment. Disease-causing organisms include bacteria, viruses, protozoans, helminths and fungi.

• Typhoid is caused by Salmonella typhi and can be diagnosed using the Widal test. Pneumonia affects alveoli, which become fluid-filled.
• Common cold is caused by rhinoviruses and affects nose and respiratory passage, not lungs.
• Malaria is caused by Plasmodium; female Anopheles mosquito is the vector. Release of haemozoin causes recurrent fever and chills.
• Amoebiasis is caused by Entamoeba histolytica; ascariasis by Ascaris; filariasis by Wuchereria.

Innate immunity is non-specific and present from birth. It includes physical barriers (skin, mucus), physiological barriers (acid in stomach, lysozyme in tears), cellular barriers (phagocytes) and cytokine barriers (interferons).

• Acquired immunity is antigen-specific and has memory. The secondary response is stronger and faster than the primary response.
• B-lymphocytes produce antibodies and mediate humoral response. T-lymphocytes mediate cell-mediated immunity and graft rejection.
• Active immunity develops after infection or vaccination. Passive immunity is immediate protection by ready-made antibodies, such as maternal antibodies or antivenom.

Allergy is an exaggerated immune response to allergens such as pollen, dust mites or animal dander. It involves release of histamine and serotonin; antihistamines, adrenaline and steroids may reduce symptoms.

Autoimmunity occurs when the immune system attacks self cells. Immunodeficiency results when immune response is weakened, as in AIDS.

HIV is a retrovirus that infects macrophages and helper T lymphocytes. Viral RNA is converted into DNA by reverse transcriptase, integrated into host DNA and produces new virus particles that progressively reduce helper T cells.

• Transmission occurs through unprotected sexual contact, contaminated blood or needles, and infected mother to child.
• HIV is not transmitted by casual social contact, sharing food, mosquito bites or physical touch.
• ELISA is commonly used to detect HIV antibodies. Prevention depends on safe sex, screened blood and sterile needles.

Cancer cells lose normal contact inhibition and divide continuously. Benign tumours remain localised; malignant tumours invade surrounding tissue and can spread through blood to distant sites, a process called metastasis.

• Carcinogens include ionising radiation, UV radiation, chemicals in tobacco smoke and some oncogenic viruses.
• Diagnosis may use biopsy and histopathology, blood/bone-marrow tests, radiography, CT or MRI.
• Treatment may include surgery, radiotherapy, chemotherapy and immunotherapy using alpha-interferon.

Repeated use of addictive drugs can produce dependence and withdrawal symptoms. Adolescents may be particularly vulnerable because of curiosity, peer pressure, stress and easy availability.

• Opioids such as morphine and heroin bind opioid receptors. Cannabinoids are obtained from Cannabis sativa. Cocaine interferes with dopamine transport.
• Intravenous drug abuse increases risk of HIV and hepatitis B transmission through shared needles.
• Supportive family environment, counselling, professional help and rehabilitation can prevent or manage abuse.

• Health is complete physical, mental and social well-being.
• Pathogens spread by air, water, food, vectors, blood and contact; prevention breaks transmission.
• Innate defence is non-specific; acquired immunity is specific and has memory.
• HIV damages helper T cells and leads to AIDS; cancer involves uncontrolled cell division.
• Drug and alcohol abuse causes physical, psychological and social harm but is preventable and treatable.