Human Health and Disease — Study Notes

1. Introduction

Health is a state of complete physical, mental and social well-being and not merely the absence of disease. The World Health Organization's definition emphasises that health is a positive concept, dependent on our lifestyle, hygiene, diet, exercise, heredity and environment. Disease can be defined as a condition in which the functioning of the body is impaired, and it may be caused by pathogens, genetic factors, nutritional deficiencies or unhealthy lifestyles. The chapter explores the different types of diseases affecting humans, their causes, symptoms and prevention.

The chapter covers infectious diseases caused by bacteria, viruses, protozoa and worms, and the immunity that protects us from them. It explains the immune system, vaccination and immunisation, cancer, AIDS, drug and alcohol abuse. An understanding of human health and disease is essential not only for examinations but also for leading a healthy life. The knowledge of how pathogens spread, how the immune system defends us, and how lifestyle choices affect health enables us to take preventive measures and make informed decisions about our well-being.

2. Common Diseases in Humans

2.1 Bacterial Diseases

• Typhoid: Caused by the bacterium Salmonella typhi. It is transmitted through contaminated food and water (faecal-oral route). Symptoms include prolonged high fever, weakness, stomach pain, constipation, headache and loss of appetite. The intestinal perforation and death may occur in severe cases. Typhoid is diagnosed by the Widal test.
• Pneumonia: Caused by bacteria like Streptococcus pneumoniae and Haemophilus influenzae. The alveoli of the lungs get filled with fluid, causing breathing difficulty and reduced oxygen intake. Symptoms include fever, chills, cough and headache. It is transmitted by inhaling droplets released by infected persons.

2.2 Protozoan Diseases

• Malaria: Caused by the protozoan Plasmodium, of which Plasmodium vivax, P. malariae and P. falciparum are common. It is transmitted by the bite of the female Anopheles mosquito. The parasite enters the red blood cells and multiplies, causing cycles of fever, chills and sweating every 48 to 72 hours.
• Amoebiasis (amoebic dysentery): Caused by the protozoan Entamoeba histolytica. It is transmitted through contaminated food and water. Symptoms include constipation, abdominal pain, cramps and stools with excess mucus and blood clots.
• Kala-azar (visceral leishmaniasis): Caused by the protozoan Leishmania donovani, transmitted by the bite of the sand fly (Phlebotomus).

2.3 Viral Diseases

• Common cold: Caused by the Rhinovirus. Symptoms include nasal congestion, sore throat, cough, headache, tiredness and watery eyes. It is transmitted by droplet infection.
• AIDS (Acquired Immuno Deficiency Syndrome): Caused by the Human Immunodeficiency Virus (HIV), which attacks the immune system.
• Dengue and Chikungunya: Transmitted by the bite of the Aedes mosquito.

2.4 Helminthic Diseases

• Ascariasis: Caused by the roundworm Ascaris, transmitted through contaminated water and food. Symptoms include abdominal pain, constipation, coughing, and internal bleeding.
• Filariasis (elephantiasis): Caused by the filarial worms Wuchereria bancrofti and Wuchereria malayi, transmitted by the bite of the female Culex mosquito. The worms live in the lymphatic system, causing chronic inflammation and blockage, leading to the swelling of the lower limbs (elephantiasis).
• Taeniasis: Caused by the tapeworm Taenia solium, transmitted through undercooked pork containing the larvae.

3. Immunity

Immunity is the ability of the body to resist infection and disease. The system that provides immunity is called the immune system. There are two main types of immunity: innate immunity and acquired immunity.

3.1 Innate Immunity

Innate immunity is the non-specific defence that we are born with. It does not distinguish between types of pathogens and acts as the first line of defence. It has four types of barriers: - Physical barriers: The skin prevents the entry of microbes, and mucus coating of the respiratory tract traps microorganisms. - Physiological barriers: Acid in the stomach, saliva in the mouth and tears in the eyes (with the enzyme lysozyme) destroy microbes. - Cellular barriers: Certain white blood cells like neutrophils, monocytes and natural killer cells, and macrophages that engulf and destroy microbes. - Cytokine barriers: Proteins like interferons are secreted by virus-infected cells and protect non-infected cells from further viral infection.

3.2 Acquired Immunity

Acquired immunity is the specific defence that develops during the lifetime of an organism. It is of two types: - Active immunity: The body produces its own antibodies after exposure to a pathogen or a vaccine. It is slow to develop but long lasting. For example, recovery from an infection, or vaccination. - Passive immunity: The body receives readymade antibodies from another source. It is immediate but short lived. For example, antibodies transferred from mother to foetus through the placenta, or in colostrum, or the injection of antitoxin for tetanus.

The cells involved in acquired immunity are B-lymphocytes (which produce antibodies) and T-lymphocytes (which help other immune cells and directly destroy infected cells). Antibodies are proteins (immunoglobulins) produced in response to antigens (foreign substances). Memory cells remember the antigen, so a second exposure produces a faster and stronger response.

3.3 Vaccination and Immunisation

Vaccination is the process of introducing a weakened or inactivated pathogen (or its antigen) into the body to generate active immunity. The principle is to expose the immune system to the antigen so that memory cells are produced, protecting against future infections. Immunisation programmes like the Expanded Programme on Immunisation (EPI) and the Pulse Polio Programme have helped eliminate smallpox and control polio. Vaccines have eradicated smallpox completely.

4. AIDS (Acquired Immuno Deficiency Syndrome)

AIDS is caused by the Human Immunodeficiency Virus (HIV), a retrovirus that contains RNA as its genetic material. HIV attacks the T-lymphocytes (helper T cells), which are essential for immune function. As the virus multiplies in these cells, it destroys them, severely weakening the immune system. The infected person becomes susceptible to many opportunistic infections like tuberculosis, pneumonia, and certain cancers.

4.1 Transmission of HIV

HIV is transmitted through: - Sexual contact with an infected person (most common route). - Transfusion of contaminated blood and blood products. - Sharing of infected needles and syringes. - From an infected mother to her child through the placenta or during childbirth.

HIV is not transmitted by casual contact, shaking hands, hugging, sharing food, or by the bite of insects or mosquitoes.

4.2 Diagnosis and Prevention of AIDS

AIDS is diagnosed by the ELISA test. The window period is the time between infection and the appearance of antibodies, during which the test may be negative. There is no cure for AIDS, but the disease can be prevented by: - Using disposable needles and syringes. - Screening blood before transfusion. - Avoiding sex with multiple partners and using condoms. - Educating people about the modes of transmission. - In India, the National AIDS Control Programme (NACO) has been initiated to control the spread of AIDS.

5. Cancer

Cancer is a disease of uncontrolled cell growth and division. It is caused by the failure of the normal mechanisms of cell division, leading to the formation of tumours. There are two types of tumours: benign tumours (which remain localised and do not spread) and malignant tumours (which invade neighbouring tissues and spread to distant sites, a process called metastasis). Cancer cells differ from normal cells in that they show uncontrolled division, do not show contact inhibition, and can spread.

5.1 Causes of Cancer

• Chemical carcinogens: Tobacco smoke (contains carcinogens like benzpyrene), certain food additives and industrial chemicals.
• Physical agents: Ionising radiations like X-rays, gamma rays and UV rays from the sun.
• Biological agents: Certain oncogenic viruses, such as the Human Papilloma Virus (cervical cancer) and hepatitis B virus.

5.2 Detection and Treatment

Cancer can be detected by biopsy, histopathological studies, blood tests, X-rays, CT scans, MRI and molecular techniques. Treatment methods include surgery (removal of the tumour), radiation therapy (using ionising radiation to kill cancer cells) and chemotherapy (using drugs to kill cancer cells). Immunotherapy, gene therapy and the use of monoclonal antibodies are newer approaches. Cancer is generally easier to treat if detected early.

6. Drugs and Alcohol Abuse

Drugs and alcohol abuse is the misuse of certain chemical substances that affect the nervous system and behaviour. These substances are often used for non-medical reasons to alter mood and behaviour, leading to addiction.

6.1 Commonly Abused Drugs

• Opioids: Drugs derived from the opium poppy, e.g., morphine and heroin. They act as depressants and relieve pain but are highly addictive.
• Cannabinoids: Chemicals present in Cannabis sativa, e.g., marijuana and hashish. They bind to receptors in the brain and alter mood and perception.
• Cocaine: An alkaloid from the coca plant, a powerful stimulant that produces euphoria but is very addictive.
• Other stimulants and hallucinogens: LSD (lysergic acid diethylamide) and barbiturates, amphetamines and benzodiazepines.

6.2 Effects of Drug Abuse

Drug abuse causes addiction, dependence and withdrawal symptoms. The effects include: - Damage to the nervous system, liver and other organs. - Increased risk of infectious diseases like AIDS and hepatitis (through needle sharing). - Mental and behavioural changes, leading to crime and accidents. - Social and economic damage to the individual, family and society.

6.3 Prevention of Drug Abuse

Drug abuse can be prevented by proper education and counselling, by strengthening individual resistance, by providing support to families, and by seeking help from de-addiction centres. The slogan "Just say no" and programmes that promote a healthy lifestyle help prevent drug abuse among youth.

Quick Revision Tables

Table 1: Common Diseases and Their Pathogens

Table 2: Types of Immunity

Table 3: Barriers of Innate Immunity

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students think malaria is transmitted by any mosquito; it is transmitted only by the bite of the female Anopheles mosquito, and filariasis by the Culex mosquito.
2. Typhoid is said to be caused by a virus; it is a bacterial disease caused by Salmonella typhi and is diagnosed by the Widal test.
3. Innate immunity is said to be specific; innate immunity is non-specific, while acquired immunity is specific.
4. Passive immunity is said to last long; passive immunity (readymade antibodies) is immediate but short lived, while active immunity is slow but long lasting.
5. HIV is said to be a DNA virus; HIV is a retrovirus with RNA as its genetic material, which uses reverse transcriptase.
6. AIDS is said to be transmitted by shaking hands or mosquito bites; HIV is not transmitted by casual contact, insect bites, hugging or sharing food.
7. All tumours are said to be cancerous; benign tumours remain localised and do not spread, while malignant tumours invade and metastasise.
8. Cancer cells are said to show contact inhibition; cancer cells do NOT show contact inhibition, which is why they continue to divide and grow over each other.

Exam Tips

1. Learn the disease-pathogen-vector triplet for every disease: malaria (Plasmodium, Anopheles), amoebiasis (Entamoeba), filariasis (Wuchereria, Culex), typhoid (Salmonella, Widal test).
2. Memorise the four barriers of innate immunity with one example each: physical (skin), physiological (gastric acid), cellular (macrophages), cytokine (interferons).
3. Distinguish active immunity (own antibodies, long lasting) from passive immunity (readymade antibodies, short lived), with one example each.
4. For AIDS remember: retrovirus, RNA genome, reverse transcriptase, attacks T-lymphocytes, ELISA test, transmitted by blood/sex/needles and mother-to-child, and NACO programme.
5. For cancer, remember the difference between benign and malignant tumours, metastasis, and the three types of carcinogens (chemical, physical, biological).
6. List the drugs under categories: opioids (morphine, heroin), cannabinoids (marijuana), cocaine (stimulant), LSD (hallucinogen).
7. Vaccination produces active immunity through memory cells; remember the smallpox eradication success story.

Conclusion

Human health and disease is a practical and life-relevant chapter that connects biology with everyday well-being. It surveys the major infectious diseases of humans, from bacterial typhoid and pneumonia to protozoan malaria, viral AIDS and helminthic filariasis, emphasising their pathogens, modes of transmission and prevention. The immune system, with its innate and acquired arms, is the body's remarkable defence network, and vaccination harnesses acquired immunity to protect entire populations. The chapter also addresses the modern epidemics of AIDS, cancer and drug abuse, which are strongly linked to lifestyle and behaviour. Understanding the causes, transmission, detection and prevention of these diseases empowers individuals to take charge of their own health. This knowledge forms the basis for the applied biotechnology chapters that follow, where these biological insights are translated into strategies for food production and human welfare.