Human Health and Diseases

Health is far more than the absence of illness. It is defined as a state of complete physical, mental and social well-being. A balanced diet, personal hygiene, regular exercise, awareness about diseases, timely immunisation, proper disposal of wastes and control of disease-carrying insects all contribute to it. Good health is not only a personal matter either. In a population it shows up as greater efficiency at work, higher productivity, longer life expectancy and lower infant and maternal mortality.

For centuries illness was blamed on evil spirits or on an imbalance of body fluids. The germ theory of disease replaced that idea with evidence: many diseases are caused by living micro-organisms. That single shift made vaccines, antibiotics and organised public health possible, and it is why modern biology begins the study of disease by naming the organism responsible.

Diseases fall into two broad groups. Infectious or communicable diseases are caused by pathogens and pass from person to person; non-infectious diseases such as cancer stay with the individual. Organisms that cause disease are called pathogens, and they are drawn from every major group of micro-organisms and parasites: bacteria, viruses, fungi, protozoans and helminths.

Typhoid is caused by the bacterium Salmonella typhi. The bacteria enter the small intestine through contaminated food and water, and from there migrate through the blood to other organs of the body. Sustained high fever of about 39 to 40 degrees Celsius, weakness, stomach pain, constipation, headache and loss of appetite are its features. In severe untreated cases the intestinal wall may be perforated. A confirmatory laboratory test for typhoid is the Widal test. A famous cook in the early twentieth century, remembered as Typhoid Mary, spread typhoid through the food she prepared even though she herself remained a carrier without the disease.

Pneumonia is caused by the bacteria Streptococcus pneumoniae and Haemophilus influenzae. These infect the alveoli of the lungs, which fill with fluid, so breathing becomes difficult. Fever, chills, cough and headache follow, and in severe cases the lips and finger nails may turn grey to bluish. A healthy person picks it up by inhaling the droplets or aerosols released by an infected person, or even by sharing glasses and utensils with one.

The common cold is caused by rhinoviruses. Note the site carefully, because it is easily confused: rhinoviruses infect the nose and the respiratory passage, but not the lungs. Nasal congestion and discharge, sore throat, hoarseness, cough, headache and tiredness usually last three to seven days. It spreads through droplets from coughs and sneezes, either inhaled directly or picked up indirectly from contaminated objects such as pens, doorknobs, books and cups.

Dengue and chikungunya are two more viral diseases you should be able to place. Both are caused by viruses and both are carried by Aedes mosquitoes, which breed in small collections of clean water in and around houses and bite mainly during the day. Because the pathogen is a virus and the carrier is a mosquito, control depends on removing breeding sites rather than on antibiotics.

Prevention at the public health level rests on hygiene at two scales. Personal hygiene means keeping the body clean and using clean drinking water, food, vegetables and fruits. Public hygiene means proper disposal of waste and excreta, periodic cleaning and disinfection of water reservoirs, pools and tanks, and standard hygiene practices in public catering. Alongside these, the discovery of antibiotics and the spread of vaccination and immunisation programmes have brought several serious diseases under control.

Malaria is caused by the protozoan Plasmodium. Several species infect humans, including Plasmodium vivax, Plasmodium malariae and Plasmodium falciparum, the last causing the most serious, malignant form of malaria. The parasite is digenetic, meaning it needs two hosts to complete its life cycle: a human being and a female Anopheles mosquito. The cycle is easiest to hold on to as a plain sequence of stages, so follow the direction of travel one step at a time.

The infective stage that a mosquito injects into a person while biting is the sporozoite. Sporozoites travel to the liver and multiply inside liver cells. From the liver the parasites move into red blood cells, multiply there, and cause the red cells to rupture. That rupture releases a toxic substance called haemozoin, and haemozoin is what produces the chill and high fever that recur every three to four days. Some parasites inside the red cells develop into gametocytes, the sexual stages. When a female Anopheles mosquito feeds on an infected person, it takes up these gametocytes along with the blood. Inside the mosquito the gametes fuse to form a zygote in the gut of the mosquito; the zygote develops further there and gives rise to sporozoites, which then move to and are stored in the mosquito's salivary glands. The next bite injects them into another human being, and the cycle begins again. Because the sexual phase and fertilisation happen inside the mosquito, the mosquito is the definitive host and the human being is the intermediate host.

Amoebiasis, also called amoebic dysentery, is caused by Entamoeba histolytica, a protozoan parasite of the large intestine. Constipation, abdominal pain and cramps, and stools with excess mucous and blood clots are its symptoms. Drinking water and food contaminated by faecal matter are the main source of infection, and houseflies act as mechanical carriers, carrying the parasite from the faeces of an infected person to food. Note the word mechanical: the housefly is only a passenger vehicle, not a host in which the parasite develops.

Ascariasis is caused by the intestinal roundworm Ascaris. Internal bleeding, muscular pain, fever, anaemia and blockage of the intestinal passage are its features. The eggs of the parasite leave the body with the faeces of an infected person and contaminate soil, water and plants. A healthy person acquires the infection by taking in contaminated water, vegetables or fruits.

Elephantiasis, or filariasis, is caused by the filarial worms Wuchereria bancrofti and Wuchereria malayi. These worms cause a slow, disfiguring, chronic inflammation of the lymphatic vessels, most often of the lower limbs, which may build up over many years; the genital organs are also often affected. The worms are transmitted to a healthy person through the bite of female mosquito vectors; Culex is the mosquito usually named for Wuchereria bancrofti.

Ringworm is one of the commonest fungal infections. It is caused by fungi of the genera Microsporum, Trichophyton and Epidermophyton, and it appears as dry, scaly lesions on the skin, nails and scalp, with intense itching. Heat and moisture help these fungi grow in skin folds such as the groin or between the toes, and infection is generally picked up from soil or from using towels, clothes or combs used by an infected person.

Because several of these diseases travel through a vector, prevention includes controlling the vector and its breeding places. That means not letting water stagnate around houses, cleaning coolers and tanks regularly, fitting doors and windows with wire mesh, using mosquito nets, spraying insecticides in ditches and drainage areas, and introducing larvivorous fishes such as Gambusia into ponds, which feed on mosquito larvae.

Immunity is the ability of the body to fight disease-causing organisms. It comes in two forms, and the split is the backbone of this chapter.

Innate immunity is non-specific and is present from birth. It does not care which pathogen has arrived and it does not improve with experience. It works through four kinds of barriers. Physical barriers are the skin, which is the main outer covering, and the mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts, which traps microbes. Physiological barriers are chemical conditions that microbes cannot survive: acid in the stomach, saliva in the mouth and tears from the eyes. Cellular barriers are certain white blood cells that swallow and destroy microbes, namely the polymorphonuclear leukocytes or neutrophils, monocytes and natural killer cells in the blood, along with macrophages in the tissues. Cytokine barriers are proteins called interferons, secreted by virus-infected cells, which protect neighbouring uninfected cells from viral infection.

Acquired immunity is pathogen specific and is characterised by memory. The first encounter with a pathogen produces a primary response of low intensity. A later encounter with the same pathogen produces a secondary or anamnestic response that is far stronger and much faster. This happens because the first meeting leaves behind memory cells that recognise the pathogen instantly.

Two kinds of lymphocytes do this work. B-lymphocytes produce an army of proteins, the antibodies, that are sent into the blood to fight the pathogen. T-lymphocytes do not themselves secrete antibodies; they help B-cells produce them, and they carry out other defensive roles of their own.

The structure of an antibody is worth setting out carefully in words. Each antibody molecule is built from four polypeptide chains: two identical shorter chains called light chains and two identical longer chains called heavy chains. Because of this the molecule is written as H2L2. The two heavy chains lie side by side and are joined to each other along part of their length, forming the stem of a Y-shaped molecule; the upper part of each heavy chain separates outwards to form the two arms of the Y, and one light chain runs alongside each arm. At the outer tip of each arm the light chain and the heavy chain together form an antigen-binding site, so one antibody molecule can grip two identical antigen sites at the same time. Several classes of antibody are produced in the body, including IgA, IgM, IgE and IgG.

Acquired immunity is further divided by the way it is obtained. In active immunity the host is exposed to antigens, either living or dead microbes or other proteins, and the host's own body makes the antibodies. Active immunity is slow and takes time to reach its full strength, but it is long lasting because memory cells are formed. A natural infection produces active immunity, and so does deliberate immunisation. In passive immunity, ready-made antibodies are supplied from outside, so protection is immediate but short-lived and no memory is formed. The two natural examples are worth memorising exactly: the yellowish colostrum secreted by the mother during the first days of feeding is rich in antibodies of the IgA class and protects the infant, and the foetus receives antibodies from the mother across the placenta during pregnancy.

Finally, acquired immunity is described by the tool it uses. Humoral immunity is antibody-mediated immunity, since the antibodies circulate in the blood and body fluids, and humor is an old word for body fluid. Cell-mediated immunity is carried out by T-lymphocytes themselves. Cell-mediated immunity is the reason the body can tell self from non-self, and therefore it is the basis of graft rejection: transplanted tissue from another person is recognised as foreign and attacked, which is why tissue matching and blood group matching are done before transplantation and why immunosuppressants are used afterwards.

The whole idea of vaccination rests on a single property of acquired immunity, its memory. In vaccination a preparation called a vaccine is introduced into the body. The vaccine contains antigenic proteins of a pathogen, or the pathogen itself in an inactivated or weakened form. It is not able to cause the disease, but the immune system cannot tell the difference between these antigens and the real thing.

Two useful things then happen. First, the body produces antibodies against these antigens, and those antibodies would neutralise the pathogen during an actual infection. Second, and more importantly, the vaccine generates memory B-cells and memory T-cells. If the real pathogen ever enters the body, these memory cells recognise it at once and the secondary response overwhelms the invader with a rapid, massive production of antibodies. This is why vaccination gives long-lasting protection while a dose of ready-made antibodies does not.

Because the body itself does the work, vaccination is a form of active immunisation. There are situations, however, where there is no time to wait for the body to build its own response. When a person is exposed to a deadly toxin, as in tetanus, preformed antibodies, called an antitoxin, are introduced directly so that protection is instant. The same logic applies to snakebite, where the preparation given contains preformed antibodies against the snake venom. Supplying antibodies made in another body in this way is passive immunisation. It acts immediately but leaves no memory behind, so it protects only for a short while.

Immunisation programmes have changed public health enormously. Smallpox has been completely eradicated from the world, and diseases such as polio, diphtheria, pneumonia and tetanus have been brought under control in large parts of the world through mass immunisation. Recombinant DNA technology has pushed this further, allowing antigenic polypeptides of a pathogen to be produced in bacteria or yeast. Vaccines made this way can be produced on a large scale and are safer, because no live pathogen is involved at any stage. The hepatitis B vaccine produced from yeast is the standard example.

An allergy is an exaggerated response of the immune system to certain antigens present in the environment. The substances that provoke it are called allergens, and the familiar ones are mites in dust, pollen grains and animal dander. The antibodies produced in response to allergens are of the IgE class. Sneezing, watery eyes, a running nose and difficulty in breathing are the usual responses. The immediate cause of these symptoms is the release of chemicals such as histamine and serotonin from mast cells. Notice the logic: the allergen itself is usually harmless, and the damage is done by the body's own overreaction to it. Modern lifestyles, with protected upbringing and reduced early exposure to a variety of antigens, are thought to be behind the rising number of allergies in children.

Autoimmunity is a different failure. The immune system normally distinguishes self from non-self, and memory-based acquired immunity depends on that distinction holding. Sometimes, for genetic and other reasons that are not fully understood, the body begins to attack its own cells. The resulting damage is called an auto-immune disease, and rheumatoid arthritis is the standard example of one.

All of this activity is housed in the lymphoid organs, the organs where lymphocytes originate, mature and multiply. They are divided into two groups, and the division is by function.

Primary lymphoid organs are the bone marrow and the thymus. These are the places where immature lymphocytes differentiate into antigen-sensitive lymphocytes. The bone marrow is the main lymphoid organ, where all blood cells including lymphocytes are produced. The thymus is a lobed organ located near the heart and beneath the breast bone; it is quite large at birth but shrinks steadily with age, and by the time puberty is reached it has become very small. Both the bone marrow and the thymus provide the micro-environments needed for the development and maturation of T-lymphocytes.

After they mature, the lymphocytes migrate to the secondary lymphoid organs: the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix. These provide the sites where lymphocytes actually meet antigens, after which they proliferate to become effector cells. The spleen is a large bean-shaped organ containing mainly lymphocytes and phagocytes; it filters the blood by trapping blood-borne micro-organisms, and it also holds a large reservoir of erythrocytes. Lymph nodes are small solid structures placed at intervals along the lymphatic system; they trap micro-organisms and other antigens that get into the lymph and tissue fluid, and the trapped antigens activate the lymphocytes present there, setting off the immune response. There is also a large amount of lymphoid tissue within the lining of the major tracts, called mucosa-associated lymphoid tissue or MALT, which makes up roughly fifty per cent of all the lymphoid tissue in the human body.

In a healthy body, cell growth and differentiation are tightly controlled and regulated. In cancer these regulatory mechanisms break down. Normal cells show a property called contact inhibition, by which contact with other cells inhibits their further growth and division. Cancer cells appear to have lost this property, so they keep dividing and pile up into masses of cells called tumours.

Tumours are of two types. Benign tumours normally remain confined to their original site, do not spread to other parts of the body and cause limited damage. Malignant tumours are masses of proliferating cells called neoplastic or tumour cells. They grow very rapidly, invading and damaging the surrounding normal tissue, and as they divide they also starve the normal cells by competing for vital nutrients. Cells shed from a malignant tumour travel through the blood, lodge at distant sites and start fresh tumours there. This property is called metastasis, and it is the most dangerous property of malignant tumours.

The conversion of a normal cell into a cancerous one may be brought about by physical, chemical or biological agents, and such agents are called carcinogens. Ionising radiations such as X-rays and gamma rays, and non-ionising radiation such as ultraviolet light, damage DNA and can lead to neoplastic transformation. The chemical carcinogens present in tobacco smoke are a major cause of lung cancer. Some viruses, called oncogenic viruses, carry genes known as viral oncogenes. Normal cells also carry genes called cellular oncogenes or proto-oncogenes, which are harmless in their normal state but, when activated under certain conditions, can transform the cell into a cancerous one.

AIDS stands for Acquired Immuno Deficiency Syndrome. Acquired means it is not inherited but picked up, immunodeficiency means a fall in the body's defensive ability, and syndrome means a group of symptoms occurring together. It is caused by the Human Immunodeficiency Virus (HIV), which is a retrovirus, that is, a virus with an RNA genome that is copied into DNA inside the host.

The routes of transmission are specific and worth listing exactly: sexual contact with an infected person, transfusion of contaminated blood and blood products, sharing of infected needles, and transmission from an infected mother to her child through the placenta. HIV does not spread through ordinary social contact such as shaking hands, hugging, sharing a meal or living alongside an infected person, and patients therefore need support rather than isolation. There is usually a long time lag, from a few months to several years, between infection and the appearance of symptoms.

The mechanism explains why the disease is so serious. After entering the body, the virus enters macrophages, where its RNA genome is used to make viral DNA with the help of the enzyme reverse transcriptase. This viral DNA becomes part of the host cell's DNA and directs the infected cell to produce virus particles, so the macrophage keeps working as a virus factory. At the same time HIV enters helper T-lymphocytes, replicates inside them and produces progeny viruses that are released into the blood and attack further helper T-lymphocytes. Repeated over time this causes a progressive fall in the number of helper T-lymphocytes, and with the coordinating cells of the immune system gone, the person becomes unable to resist infections that a healthy body would easily overcome, including those caused by bacteria, viruses, fungi and parasites. Infection is detected by a test called ELISA. Prevention is handled at the level of public health programmes: making blood in blood banks safe, ensuring that only disposable needles and syringes are used, controlling drug abuse and running large-scale education campaigns.

Adolescence means both a period and a process: the stage during which a child matures in attitudes and beliefs and becomes able to take part in society as an adult. It is usually taken as the years between about twelve and eighteen, and it is best thought of as a bridge linking childhood and adulthood. Adolescence is accompanied by many biological and behavioural changes at once, which is why it is described as a vulnerable phase of mental and psychological development.

Several ordinary features of this stage explain the vulnerability. Curiosity, a need for adventure and excitement, and a wish to experiment are normal at this age but can push a young person towards trying things that are harmful. Peer pressure adds to it, as does the wish to be seen as grown up or as part of a group. Pressure to perform in school or in examinations can add stress that a young person may not know how to handle. An unstable or unsupportive family environment and the influence of what is seen in television, films and social media are further factors that make young people more likely to take up harmful habits.

The general effects of drug and alcohol misuse show themselves first as a set of warning signs. A drop in academic performance, unexplained absence from school or college, loss of interest in personal hygiene, withdrawal and isolation, depression and fatigue, aggressive and rebellious behaviour, deteriorating relationships with family and friends, loss of interest in hobbies, and changes in sleeping and eating habits with fluctuations in weight and appetite are all recognised signs. The harm is not limited to the individual. Families are affected through anxiety and financial strain, and the wider social effects include damage to relationships and to the young person's future prospects.

Prevention carries the real weight here, and it sits at the level of families, schools and society rather than at the level of the individual alone. Young people should be helped to avoid undue peer pressure, since every child has their own capacity and pace, and parents and teachers should not push a child beyond it. Education and counselling matter: teaching young people to face problems and stresses, and to channel their energy into studies, sports, music, reading and other productive activities. Parents and peers can offer support at the right moment, especially in the first phase of a problem. The last element is recognising the danger signs early and turning to qualified professional and medical help, which is treated as a sensible step rather than an admission of failure. With the right help, a young person can be brought back to a normal, healthy life.