Human Health and Disease

For a very long time, health was considered a state of balance between certain bodily "humours." The ancient Greeks, led by Hippocrates, along with practitioners of the Indian Ayurveda system, believed that diseases arose from an imbalance of these humours. William Harvey's discovery of blood circulation, combined with later experiments using thermometers, helped disprove this notion. Modern biology has shown us that the mind influences our immune system through neural and endocrine pathways, and that the immune system is central to maintaining health.

Health is affected by three broad categories of factors:

• Genetic disorders — conditions present from birth, inherited from parents or arising from developmental defects.
• Infections — caused by pathogens such as bacteria, viruses, fungi, protozoans, and helminths.
• Lifestyle — including diet, hydration, rest, exercise, personal habits, and exposure to harmful substances.

7.1 Common Diseases in Humans

A wide range of organisms — bacteria, viruses, fungi, protozoans, and helminths — can cause disease in humans. Such disease-causing organisms are called pathogens. Most parasites are pathogens because they cause harm to the host by living in or on them. Pathogens enter the body through various routes, multiply, and interfere with normal vital activities, resulting in morphological and functional damage. They must also adapt to the host's internal environment — for instance, gut pathogens must survive the acidic pH of the stomach and resist digestive enzymes.

Typhoid

Salmonella typhi is the pathogenic bacterium responsible for typhoid fever. These pathogens generally enter the small intestine through contaminated food and water, and then migrate to other organs via the bloodstream. Common symptoms include sustained high fever (39–40 °C), weakness, stomach pain, constipation, headache, and loss of appetite. In severe cases, intestinal perforation and death can occur. The disease can be confirmed by the Widal test.

Pneumonia

Bacteria such as Streptococcus pneumoniae and Haemophilus influenzae cause pneumonia, which infects the alveoli (air-filled sacs) of the lungs. The alveoli become filled with fluid, leading to severe respiratory distress. Symptoms include fever, chills, cough, and headache. In severe cases, the lips and fingernails may turn grayish-blue. Infection occurs by inhaling droplets or aerosols from an infected person, or by sharing glasses and utensils.

Common Cold

Rhinoviruses cause one of the most infectious human ailments — the common cold. They infect the nose and respiratory passage but not the lungs. The disease is characterised by nasal congestion and discharge, sore throat, hoarseness, cough, headache, and tiredness, typically lasting 3–7 days. Droplets from coughs or sneezes of an infected person are inhaled directly, or infection spreads through contaminated objects such as pens, books, cups, doorknobs, computer keyboards, and mice.

Malaria

Plasmodium, a tiny protozoan, is responsible for malaria — a disease that humans have been battling for centuries. Different species cause different types of malaria: P. vivax, P. malariae, and P. falciparum. Malignant malaria caused by P. falciparum is the most serious and can be fatal.

Life cycle of Plasmodium: Plasmodium enters the human body as sporozoites (the infectious form) through the bite of an infected female Anopheles mosquito. The parasites multiply within liver cells, then attack red blood cells (RBCs), causing their rupture. The rupture of RBCs releases a toxic substance called haemozoin, which is responsible for the chill and high fever that recur every three to four days.

When a female Anopheles bites an infected person, the parasites enter the mosquito and undergo further development. They multiply to form sporozoites stored in the mosquito's salivary glands. When this mosquito bites another human, the sporozoites are introduced, completing the cycle. Importantly, Plasmodium requires two hosts — the human and the mosquito.

Amoebiasis

Entamoeba histolytica is a protozoan parasite inhabiting the large intestine that causes amoebiasis (amoebic dysentery). Symptoms include constipation, abdominal pain and cramps, and stools containing excess mucous and blood clots. Houseflies act as mechanical carriers, transmitting the parasite from the faeces of infected persons to food and food products. Contaminated drinking water and food are the main sources of infection.

Ascariasis

Ascaris, the common roundworm, is a helminth parasite pathogenic to humans. It causes ascariasis, with symptoms including internal bleeding, muscular pain, fever, anaemia, and blockage of the intestinal passage. The eggs of the parasite are excreted in the faeces of infected persons, contaminating soil, water, and plants. A healthy person acquires this infection through contaminated water, vegetables, and fruits.

Elephantiasis (Filariasis)

Wuchereria (W. bancrofti and W. malayi), filarial worms, cause a slowly developing chronic inflammation of the organs in which they live for many years — usually the lymphatic vessels of the lower limbs. The resulting disease is called elephantiasis or filariasis. The genital organs are also often affected, resulting in gross deformities. The pathogens are transmitted to a healthy person through the bite of female mosquito vectors.

Ringworm

Many fungi belonging to the genera Microsporum, Trichophyton, and Epidermophyton are responsible for ringworm — one of the most common infectious diseases. Dry, scaly lesions appear on the skin, nails, and scalp, accompanied by intense itching. Heat and moisture help these fungi thrive, which is why they flourish in skin folds such as those in the groin or between the toes. Ringworm is generally acquired from soil or by using towels, clothes, or combs of infected individuals.

Prevention and Control of Infectious Diseases

Maintenance of personal and public hygiene is critical. Personal hygiene includes keeping the body clean and consuming clean drinking water, food, vegetables, and fruits. Public hygiene includes proper disposal of waste and excreta, periodic cleaning and disinfection of water reservoirs, pools, cesspools, and tanks, and observing standard hygiene practices in public catering.

For air-borne diseases like pneumonia and common cold, close contact with infected persons or their belongings should be avoided. For vector-borne diseases like malaria and filariasis, the most important measure is to control or eliminate the vectors and their breeding places. This includes avoiding stagnation of water, regular cleaning of household coolers, using mosquito nets, introducing fish like Gambusia in ponds (which feed on mosquito larvae), and spraying insecticides in ditches, drainage areas, and swamps. Doors and windows should be fitted with wire mesh to prevent mosquito entry. These precautions have become especially important in light of widespread outbreaks of vector-borne diseases like dengue and chikungunya caused by Aedes mosquitoes.

7.2 Immunity

Every day we are exposed to a large number of infectious agents, yet only a few exposures result in disease. This is because the body can defend itself from most foreign agents. This overall ability of the host to fight disease-causing organisms, conferred by the immune system, is called immunity.

Immunity is of two broad types: (i) Innate Immunity and (ii) Acquired Immunity.

7.2.1 Innate Immunity

Innate immunity is a non-specific type of defence present from birth. It works by providing different types of barriers to the entry of foreign agents into the body. Innate immunity consists of four types of barriers:

7.2.2 Acquired Immunity

Acquired immunity is pathogen-specific and characterised by memory. When the body encounters a pathogen for the first time, it produces a primary response of low intensity. Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response, because the body retains a memory of the first encounter.

The primary and secondary immune responses are carried out by two special types of lymphocytes: B-lymphocytes and T-lymphocytes.

• B-lymphocytes produce an army of proteins in response to pathogens. These proteins are called antibodies. Each antibody molecule has four peptide chains — two short light chains and two longer heavy chains, represented as H₂L₂. Different types of antibodies are produced in our body: IgA, IgM, IgE, IgG, etc. Because these antibodies are found in blood, this response is also called a humoral immune response (antibody-mediated immunity).
• T-lymphocytes do not secrete antibodies but help B cells to produce them. They mediate cell-mediated immune response (CMI), which is responsible for graft rejection during organ transplantation and for differentiating "self" from "non-self."

7.2.3 Active and Passive Immunity

• Active immunity — When a host is exposed to antigens (living or dead microbes or other proteins), antibodies are produced in the host body. This type of immunity is slow and takes time to give its full effective response. Injecting microbes deliberately during immunisation, or natural infection, induces active immunity.
• Passive immunity — When ready-made antibodies are directly given to protect the body against foreign agents. For example, the yellowish fluid colostrum secreted by the mother during the initial days of lactation contains abundant antibodies (IgA) to protect the infant. The foetus also receives some antibodies from the mother through the placenta during pregnancy.

7.2.4 Vaccination and Immunisation

The principle of immunisation or vaccination is based on the property of memory of the immune system. In vaccination, a preparation of antigenic proteins of the pathogen, or inactivated/weakened pathogen (vaccine), is introduced into the body. The antibodies produced against these antigens neutralise the pathogenic agents during actual infection. The vaccines also generate memory B- and T-cells that recognise the pathogen quickly on subsequent exposure.

In many children, some infectious diseases like polio, diphtheria, pertussis, and tetanus are prevented through immunisation. In a similar manner, a vaccine against the rabies virus (developed by Louis Pasteur) is also administered prophylactically, i.e., before being bitten by a mad dog, since the disease takes a long time to develop. Most of these vaccines are given in the form of injection. Inoculation with attenuated pathogens is also being used to develop immunity against diseases such as cholera and typhoid. The oral vaccine for polio was developed by Sabin. Newer vaccines being developed use DNA technology — these vaccines are safer because they do not contain any dead pathogen.

7.2.5 Allergies

The exaggerated response of the immune system to certain agents called allergens is called allergy. The substances or allergens responsible for an allergic reaction — such as dust, pollen, animal dander, cosmetics, or certain food items like seafood — are generally perceived as foreign bodies by our immune system. The antibodies produced are of the IgE type.

Common symptoms of allergy include sneezing, watery eyes, runny nose, and difficulty in breathing. Allergy is treated by drugs called antihistamines, and steroids. Corticosteroids quickly reduce the symptoms of allergy. Modern-day lifestyle has resulted in lowered immunity and increased sensitivity to allergens — more and more children in metro cities of India suffer from allergies and asthma due to environmental sensitivity. This could be linked to the overly protected environment provided early in life.

7.2.6 Auto Immunity

Memory-based acquired immunity in higher vertebrates evolved based on the ability to differentiate foreign organisms from self-cells. Sometimes, due to genetic and other unknown reasons, the body attacks its own self-cells. This results in damage to the body and is called auto-immune disease. Rheumatoid arthritis, which affects many people in our society, is a well-known example of an auto-immune disease.

7.2.7 Immune System in the Body

The human immune system consists of lymphoid organs, tissues, cells, and soluble molecules like antibodies. The immune system uniquely recognises foreign antigens, responds to them, and remembers them. It also plays an important role in allergic reactions, auto-immune diseases, and organ transplantation.

Lymphoid organs are the organs where origin, maturation, and proliferation of lymphocytes occur:

There is also mucosa-associated lymphoid tissue (MALT) located within the lining of the major tracts (respiratory, digestive, and urogenital tracts). It constitutes about 50 per cent of the lymphoid tissue in the human body.

7.3 AIDS

The word AIDS stands for Acquired Immuno Deficiency Syndrome. This means deficiency of the immune system, acquired during the lifetime of an individual — indicating it is not a congenital disease. "Syndrome" means a group of symptoms. AIDS was first reported in 1981 and in the last twenty-five years, it has spread worldwide, killing more than 25 million people.

Causative Agent

AIDS is caused by the Human Immunodeficiency Virus (HIV), a member of a group of viruses called retroviruses, which have an envelope enclosing the RNA genome.

Transmission

HIV infection is generally transmitted through:

• Sexual contact with an infected person
• Transfusion of contaminated blood and blood products
• Sharing infected needles, as in the case of intravenous drug abusers
• From infected mother to child through the placenta

People at high risk include individuals with multiple sexual partners, drug addicts who use drugs intravenously, individuals who require repeated blood transfusions, and children born to HIV-infected mothers. Importantly, HIV/AIDS is not spread by mere touch or physical contact — it spreads only through body fluids. Therefore, HIV/AIDS-infected persons should not be isolated from family and society.

Life Cycle of HIV

After entering the body, the virus enters macrophages, where the RNA genome replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into the host cell's DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus and thus act as an HIV factory.

Simultaneously, HIV enters helper T-lymphocytes (T_H), replicates, and produces progeny viruses. These progeny viruses released in the blood attack other helper T-lymphocytes. This cycle is repeated, leading to a progressive decrease in the number of helper T-lymphocytes in the body.

During this period, the person suffers from bouts of fever, diarrhoea, and weight loss. As helper T-lymphocytes decline, the person becomes susceptible to infections that could otherwise be overcome — infections caused by bacteria (especially Mycobacterium), viruses, fungi, and even parasites like Toxoplasma. The patient becomes so immunodeficient that he or she cannot protect against these infections. There is typically a time-lag of 5–10 years between infection and the appearance of AIDS symptoms.

Diagnosis and Treatment

A widely used diagnostic test for AIDS is the Enzyme-Linked Immunosorbent Assay (ELISA). Treatment of AIDS with anti-retroviral drugs is only partially effective — they can only prolong the life of the patient but cannot prevent death, which is ultimately inevitable.

Prevention

As AIDS has no cure, prevention is the best option. In our country, the National AIDS Control Organisation (NACO) and various NGOs work to educate people about AIDS. WHO has started a number of programmes including:

• Making blood from blood banks safe from HIV
• Ensuring use of only disposable needles and syringes in hospitals
• Free distribution of condoms
• Controlling drug abuse
• Advocating safe sex
• Promoting regular HIV check-ups in susceptible populations

7.4 Cancer

Cancer is one of the most dreaded diseases of human beings and is a major cause of death worldwide. The mechanisms underlying the development of cancer — called oncogenic transformation — its treatment, and control have been among the most intensely studied areas in biology and medicine.

Nature of Cancer

In our body, cell growth and differentiation are highly controlled and regulated. In cancer cells, there is a breakdown of these regulatory mechanisms. Normal cells show a property called contact inhibition — contact with other cells inhibits their uncontrolled growth. Cancer cells lose this property. As a result, cancerous cells continue to divide, giving rise to masses of cells called tumours.

Cells sloughed from malignant tumours reach distant sites through the blood and wherever they lodge, they start a new tumour. This property, called metastasis, is the most feared property of malignant tumours.

Causes of Cancer

Transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical, or biological agents. These agents are called carcinogens:

• Ionising radiations like X-rays and gamma rays, and non-ionising radiations like UV, cause DNA damage leading to neoplastic transformation.
• Chemical carcinogens present in tobacco smoke have been identified as a major cause of lung cancer.
• Oncogenic viruses — cancer-causing viruses that carry genes called viral oncogenes.
• Cellular oncogenes (c-onc) or proto-oncogenes — several genes identified in normal cells which, when activated under certain conditions, lead to oncogenic transformation.

Cancer Detection and Diagnosis

Early detection is essential for successful treatment. Detection methods include:

• Biopsy and histopathological studies — a piece of suspected tissue is cut into thin sections, stained, and examined under a microscope by a pathologist.
• Blood and bone marrow tests — for increased cell counts in the case of leukemias.
• Radiography (X-rays), CT (computed tomography), and MRI (magnetic resonance imaging) — to detect cancers of internal organs. CT uses X-rays to generate a 3D image of internal structures; MRI uses strong magnetic fields and non-ionising radiations to detect pathological and physiological changes.

Techniques like molecular biology are now applied to detect genes in individuals with inherited susceptibility to certain cancers. Identification of such predisposing genes may help in prevention — by advising individuals to avoid exposure to specific carcinogens.

Treatment of Cancer

The common approaches for treatment of cancer are:

• Surgery — physical removal of the tumour.
• Radiotherapy — tumour cells are irradiated lethally, taking care of surrounding normal tissues.
• Chemotherapy — various chemotherapeutic drugs are used to kill cancerous cells. Some are specific for particular tumours, though most have side effects like hair loss and anaemia.

Tumour cells have been shown to avoid detection and destruction by the immune system. Therefore, patients are given substances called biological response modifiers such as α-interferon, which activates the immune system and helps destroy the tumour. Most cancers are treated by a combination of surgery, radiotherapy, and chemotherapy.

7.5 Drugs and Alcohol Abuse

Surveys and statistics show that the use of drugs and alcohol has been on the rise, especially among the youth. This is a cause of serious concern as it results in many harmful effects. Proper education and guidance can enable young people to safeguard themselves against these dangerous behaviour patterns and follow healthy lifestyles.

The drugs most commonly abused are opioids, cannabinoids, and coca alkaloids. The majority are obtained from flowering plants; some are obtained from fungi.

Opioids

Opioids are drugs that bind to specific opioid receptors present in the central nervous system and gastrointestinal tract. Heroin, commonly called smack, is chemically diacetylmorphine — a white, odourless, bitter crystalline compound. It is obtained by acetylation of morphine, which is extracted from the latex of the poppy plant Papaver somniferum. Generally taken by snorting or injection, heroin is a depressant that slows down body functions.

Cannabinoids

Cannabinoids are a group of chemicals that interact with cannabinoid receptors present principally in the brain. Natural cannabinoids are obtained from the inflorescences of the plant Cannabis sativa. The flower tops, leaves, and resin of the cannabis plant are used in various combinations to produce marijuana, hashish, charas, and ganja. Generally taken by inhalation and oral ingestion, these are known for their effects on the cardiovascular system. Cannabinoids are also being abused by some sportspersons.

Coca Alkaloid (Cocaine)

Cocaine is obtained from the coca plant Erythroxylum coca, native to South America. It interferes with the transport of the neurotransmitter dopamine. Cocaine, commonly called coke or crack, is usually snorted. It has a potent stimulating action on the central nervous system, producing a sense of euphoria and increased energy. Excessive dosage causes hallucinations. Other well-known plants with hallucinogenic properties include Atropa belladonna and Datura.

Medicinal Drugs That Are Abused

Drugs like barbiturates, amphetamines, and benzodiazepines — normally used as medicines to help patients cope with mental illnesses like depression and insomnia — are often abused. Morphine is a very effective sedative and painkiller, useful in patients who have undergone surgery. Several plants, fruits, and seeds having hallucinogenic properties have been used for hundreds of years in folk medicine, religious ceremonies, and rituals. When taken for purposes other than medicinal use, or in amounts/frequency that impair one's physical, physiological, or psychological functions, it constitutes drug abuse.

Tobacco

Smoking also paves the way to hard drugs. Tobacco has been used for more than 400 years — smoked, chewed, or used as snuff. It contains a large number of chemical substances including nicotine, an alkaloid. Nicotine stimulates the adrenal gland to release adrenaline and nor-adrenaline into blood circulation, both of which raise blood pressure and increase heart rate. Smoking is associated with increased incidence of cancers of the lung, urinary bladder, and throat, as well as bronchitis, emphysema, coronary heart disease, and gastric ulcer. Tobacco chewing is associated with increased risk of cancer of the oral cavity. Smoking increases carbon monoxide (CO) content in blood and reduces the concentration of haem-bound oxygen, causing oxygen deficiency in the body.

7.5.1 Adolescence and Drug/Alcohol Abuse

Adolescence is both "a period" and "a process" during which a child becomes mature in terms of attitudes and beliefs for effective participation in society. The period between 12–18 years may be thought of as the adolescence period — a bridge linking childhood and adulthood. Adolescence is accompanied by several biological and behavioural changes, making it a very vulnerable phase of mental and psychological development.

Common causes that motivate youngsters towards drug and alcohol use include:

• Curiosity, need for adventure and experimentation — the child's natural curiosity motivates experimentation, complicated further by perceived "benefits" of alcohol or drug use.
• Stress — from pressures to excel in academics or examinations.
• Perception of "cool" — the notion among youth that it is progressive to smoke, use drugs or alcohol, promoted by television, movies, newspapers, and the internet.
• Unstable or unsupportive family structures and peer pressure.

7.5.2 Addiction and Dependence

Because of perceived benefits, drugs are frequently used repeatedly. The most important thing one fails to realise is the inherent addictive nature of alcohol and drugs. Addiction is a psychological attachment to certain effects — such as euphoria and a temporary feeling of well-being — associated with drugs and alcohol. These drive people to take them even when not needed, or even when their use becomes self-destructive.

With repeated use, the tolerance level of receptors in the body increases. Consequently, the receptors respond only to higher doses, leading to greater intake and addiction. Even a single use of these drugs can be a fore-runner to addiction. The addictive potential pulls the user into a vicious circle of regular use (abuse) from which escape is difficult.

Dependence is the tendency of the body to manifest a characteristic and unpleasant withdrawal syndrome if regular doses of drugs/alcohol are abruptly discontinued. This is characterised by anxiety, shakiness, nausea, and sweating, which may be relieved when use is resumed. In some cases, withdrawal symptoms can be severe and even life-threatening, requiring medical supervision.

7.5.3 Effects of Drug/Alcohol Abuse

The immediate adverse effects of drugs and alcohol abuse manifest as reckless behaviour, vandalism, and violence. Excessive doses may lead to coma and death due to respiratory failure, heart failure, or cerebral haemorrhage. Combining drugs or taking them along with alcohol generally results in overdosing and even death.

Warning signs of drug and alcohol abuse among youth include:

• Drop in academic performance
• Unexplained absence from school/college
• Lack of interest in personal hygiene
• Withdrawal, isolation, depression, fatigue
• Aggressive and rebellious behaviour
• Deteriorating relationships with family and friends
• Loss of interest in hobbies
• Change in sleeping and eating habits
• Fluctuations in weight and appetite

Far-reaching implications include: turning to stealing to fund drug purchases; causing mental and financial distress to family and friends; and increased risk of acquiring serious infections like AIDS and Hepatitis B through shared needles. Both AIDS and Hepatitis B are chronic, ultimately fatal infections that can be transmitted through sexual contact or infected blood.

Alcohol use during adolescence may also lead to heavy drinking in adulthood. Chronic use of drugs and alcohol damages the nervous system and liver (cirrhosis). Use during pregnancy adversely affects the foetus.

Another misuse involves sportspersons using narcotic analgesics, anabolic steroids, diuretics, and certain hormones to increase muscle strength and bulk and promote aggressiveness, thereby enhancing athletic performance.

7.5.4 Prevention and Control

The age-old adage "prevention is better than cure" holds true here. Habits like smoking, taking drugs, or drinking alcohol are more likely to be taken up at a young age, particularly during adolescence. It is best to identify situations that may push an adolescent towards drug or alcohol use and take remedial measures well in time. Parents and teachers have a special responsibility in this regard. Parenting that combines high levels of nurturance with consistent discipline has been associated with lowered risk of substance abuse.

• Avoid undue peer pressure — Every child has his or her own choice and personality, which should be respected and nurtured. A child should not be pushed unduly to perform beyond his or her threshold limits — be it studies, sports, or other activities.
• Education and counselling — Educating and counselling the child to face problems and stresses, and to accept disappointments and failures as a part of life. Channelising the child's energy into healthy pursuits like sports, reading, music, yoga, and other extracurricular activities.
• Seeking help from parents and peers — Help from parents and peers should be sought immediately so they can guide appropriately. Close and trusted friends can also help the young person vent feelings of anxiety and guilt.
• Looking for danger signs — Alert parents and teachers need to look for and identify the danger signs discussed above. Friends who find someone using drugs or alcohol should not hesitate to bring this to the notice of parents or teachers.
• Seeking professional and medical help — A lot of help is available in the form of highly qualified psychologists, psychiatrists, and de-addiction and rehabilitation programmes. With such help, the affected individual — with sufficient effort and willpower — can get rid of the problem completely and lead a perfectly normal and healthy life.

7.6 Summary