Health and Disease

Being free of illness is one of the most precious things in life, yet disease is a constant challenge. Science helps us understand what health really is, why we fall ill, how diseases spread, and, most importantly, how to prevent them. This knowledge of health and disease is vital both personally and for public policy, and is highly relevant to general studies.

Health is more than just not being sick. It is a state of complete physical, mental and social well-being. A healthy person is one who can function well in body and mind and live well within their community.

Health therefore depends not only on the individual but on the surroundings: clean water, good food, proper sanitation, decent housing, and a peaceful social environment. Public health is a shared responsibility, not just a private matter.

Diseases fall into two broad groups:

• Infectious (communicable) diseases are caused by micro-organisms (pathogens) such as bacteria, viruses, fungi and protozoa, and can spread from one person to another. Examples include tuberculosis, malaria, the common cold and COVID-19.
• Non-infectious (non-communicable) diseases do not spread between people. They arise from internal causes, lifestyle or genetics. Examples are cancer, diabetes and heart disease.

It helps to know which kind of microbe causes a familiar disease.

• Malaria: caused by a protozoan, the Plasmodium parasite.
• Poliomyelitis (polio): caused by a virus.
• Tuberculosis (TB): caused by a bacterium.
• Ringworm and athlete's foot: caused by fungi. Athlete's foot is a dermatophyte fungus that infects the skin between the toes. It is not a bacterium, a protozoan or a worm.
• Dengue: a viral disease, not a protozoan one. It is spread by mosquitoes. Its signs include pain behind the eyes (retro-orbital pain), a skin rash, and bleeding from the nose and gums.

Animals fall ill from the same broad groups of microbe. Among the diseases of milch (milk-giving) cattle and other livestock:

• Foot-and-mouth disease and cowpox are viral and highly contagious.
• Anthrax and black quarter are caused by bacteria.

Infectious diseases spread through several means of transmission:

• through the air (coughs and sneezes), as with the cold, flu and tuberculosis,
• through water and food contaminated with germs, as with cholera and typhoid,
• through direct contact, as with many skin diseases,
• through carriers (vectors) such as mosquitoes, which spread malaria and dengue, and
• through bodily fluids, as with HIV.

Knowing the route of spread is the key to stopping a disease.

Several infections cross more than one of these routes, which is why they spread so easily.

• Zika: the Zika virus is carried by the same Aedes mosquito that spreads dengue. It can also pass through sexual contact, so it is both vector-borne and sexually transmissible.
• Chikungunya: this viral fever is purely vector-borne. It spreads through the bite of an infected mosquito and does not pass through blood or shared needles.

Some pathogens travel in blood. They spread through transfusion of infected blood, shared injecting needles, and unsterile tattoo or piercing needles.

• HIV: spread by bodily fluids and blood. Several specifics matter. Male-to-female transmission is more likely than female-to-male. A co-existing sexually transmitted infection raises the risk of catching HIV. The risk from a unit of infected transfused blood is far higher than from a single needle-stick injury. A mother can pass HIV to her child during pregnancy, during childbirth, and through breastfeeding.
• Hepatitis B: a viral infection of the liver. It is blood-borne and spreads much like HIV, including through contaminated tattoo needles. It is far more infectious than HIV. Long-term infection can lead to liver cancer.

Microbes do not always float freely. Many gather into a biofilm, a sticky community of cells fixed to a surface. Biofilms build up on medical implants such as catheters and artificial joints, and on food and food-processing surfaces. Cells inside a biofilm are hard to kill and can show strong resistance to antibiotics.

Plant pathogens have their own routes of travel. They move from plant to plant by wind, by rain splash, by wind-blown dust, and by birds and insects. This is why a crop disease can sweep across a whole field once it takes hold.

Most infectious disease can be prevented, which is far better than curing it. Prevention works at two levels:

• General measures: clean drinking water, good sanitation, hygienic food, proper nutrition and clean surroundings stop germs from spreading.
• Specific measures: immunisation (vaccination), which trains the body's immune system to fight a particular disease before it strikes.

Vaccines have wiped out or controlled deadly diseases like smallpox and polio. Public health programmes that provide clean water, sanitation and immunisation are among the most powerful tools for improving a nation's well-being.

A vaccine is easy to design when a disease is caused by one stable germ. Malaria is hard because it is caused by several species of Plasmodium, and the parasite keeps changing the antigens on its surface. This antigenic variability lets it dodge the immune system, which is why a fully effective malaria vaccine has been so slow to arrive.

Hepatitis B and Hepatitis C are two viral infections of the liver. A few contrasts are often tested.

• A vaccine exists for Hepatitis B, but there is no vaccine for Hepatitis C.
• Hepatitis B is transmitted much like HIV, through blood and bodily fluids.
• Both Hepatitis B and C are more widespread than HIV.
• Both can stay without symptoms for many years before the damage shows.

Vaccines train the immune system in different ways. Several types featured in the response to COVID-19.

• Inactivated-pathogen vaccines use a killed germ. COVAXIN is of this type.
• Viral-vector vaccines use a harmless virus to carry a target gene into the body. Covishield is a viral-vector vaccine, not an mRNA vaccine. Sputnik V is also vector-based.
• Recombinant vector vaccines are made by genetic engineering. They use a bacterium or virus as the vector to deliver the antigen.
• Conjugate vaccines: the pneumococcal conjugate vaccine guards against pneumonia, meningitis and sepsis caused by pneumococcal bacteria. By preventing these infections it lowers the need for antibiotics, though, like all vaccines, it can have side effects.

The pathogens that cause infection are not all alike. They divide into a few main groups, and the differences between them matter for diagnosis and treatment.

Bacteria and fungi are living cells. They can be grown outside the body on an artificial (synthetic) medium, a nutrient jelly or broth prepared in a laboratory. A virus is different. It is not a full cell and lacks the enzymes needed to make its own energy. A virus is an obligate intracellular parasite: it can multiply only inside a living host cell, so it cannot be cultured on a synthetic medium alone.

Viruses are also far more wide-ranging than students often assume:

• They infect all cellular life: viruses that attack bacteria are called bacteriophages, others infect fungi, and many infect plants.
• They thrive in the sea: ocean water is full of viruses, so the idea that none survive there is false.
• They alter the host cell: many viruses change the transcriptional activity of the cell they invade, hijacking it to make new virus particles.

Two virus families appear often in exams. Adenoviruses carry double-stranded DNA and cause some cases of the common cold. Retroviruses carry single-stranded RNA and include HIV, which causes AIDS.

A few special microbe facts are worth fixing in memory:

• Extremophiles: some microbes grow above the boiling point of water (in hot springs and deep-sea vents), some below the freezing point, and some in highly acidic conditions with a pH below 3.
• Probiotics: these are live bacteria and yeasts. They occur naturally in the human gut, and many strains help digest milk sugars (lactose).
• Aflatoxins: these toxins form on poorly stored cereal grains and oilseeds. They are produced by moulds (a type of fungus, mainly Aspergillus) and are not destroyed by ordinary cooking.
• Multi-drug resistance: pathogens become resistant mainly because people take incorrect or incomplete doses of antibiotics, drugs that kill bacteria, and because antibiotics are overused in livestock farming.

Microbes also turn industrial waste into useful products. When sugar cane is milled, two by-products remain.

• Bagasse: the fibrous pulp left after the juice is pressed out. It is burned as a biomass fuel to power the mill.
• Molasses: the thick syrup left after the sugar crystals are removed. Yeast ferments it into ethanol (alcohol). Molasses is not used to make synthetic fertilisers.

Different diseases strike different organs and tissues. Knowing the target site helps explain the symptoms.

• Malaria: the Plasmodium parasite destroys red blood cells.
• Filaria (filariasis): the worm blocks the lymph nodes and lymphatic vessels, causing swelling.
• Encephalitis: the infection inflames the brain.
• Leukaemia: this blood cancer arises in the bone marrow, where blood cells are made.

Diabetes mellitus is a common disorder of adults. Its hallmarks are a high sugar level in the blood together with a low level of insulin, the hormone that lets cells absorb glucose. When insulin is deficient, glucose builds up in the blood instead of being used.

Some diseases are inherited through genes rather than caught from outside.

A sex-linked disorder is carried on the sex chromosomes, usually the X chromosome. Haemophilia, the bleeding disorder once common in European royalty and so called "royal haemophilia", is X-linked, as is colour blindness. By contrast, conditions such as Tay-Sachs disease and cystic fibrosis are autosomal, and hypertension is multifactorial.

A second group involves the mitochondria, the tiny energy factories inside cells. Mitochondria are passed down only from the mother, so mitochondrial diseases are inherited from the mother and not the father. Mitochondrial replacement therapy (MRT), a technique that swaps in healthy donor mitochondria, can prevent a mother from passing such a disease to her child. It can be carried out either before or after in-vitro fertilisation of the egg.

Some illnesses come not from germs but from missing nutrients or harmful foods.

A shortage of a particular vitamin causes a specific deficiency disease:

• Vitamin C: its lack causes scurvy, marked by bleeding gums.
• Vitamin D: its lack causes rickets, which softens and bends the bones of children.
• Vitamin A: its lack causes night blindness. Note that night blindness is not caused by Vitamin E.

Lathyrism is a different kind of food disorder. It is a crippling disease of the nervous system caused by eating large amounts of kesari dal (Lathyrus sativus, the grass pea), which contains a neurotoxic amino acid.

A few facts about how the healthy body works are frequently tested.

Nitric oxide is a gas made inside the body by the lining of blood vessels. It is a vasodilator: it widens the blood vessels and so increases blood flow. Do not confuse it with nitrous oxide (laughing gas) or with the polluting oxides of nitrogen.

The contraction of muscles depends on several minerals working together. Calcium, sodium and iron are all required: calcium triggers the contraction, sodium carries the nerve signal that starts it, and iron, in the muscle pigment myoglobin, helps supply oxygen.

The right medicine depends on the cause of the disease.

Antibiotics act only against bacteria. They have no role in the primary treatment of viral diseases. Swine flu, caused by the H1N1 influenza virus, is therefore not treated with antibiotics. Because it spreads from human to human, culling pigs is not needed to control it, and the whole population is susceptible since few people have prior immunity.

Some drugs serve more than one disease. Rifampicin is a broad antimicrobial drug used in the multi-drug regimens for both tuberculosis and leprosy.

Modern biotechnology has added powerful new tools to medicine.

• Monoclonal antibodies: these are man-made proteins built to bind to one specific antigen. By locking onto that target they support the immune response, and they are used to treat viral infections such as the Nipah virus.
• Stem cells: these unspecialised cells can renew themselves in the laboratory almost indefinitely and can grow into many tissue types. They can be obtained from several sources, not just from embryos and not from mammals only, and they are used both to screen new drugs and in medical therapies.
• Nanotechnology: it makes targeted drug delivery possible, sending medicine straight to diseased cells, and it contributes to gene therapy. Carbon nanotubes can carry drugs and antigens, work in biochemical sensors, and some types are biodegradable, though they cannot yet be made into working blood capillaries.
• Hydrogels: these water-holding gels are used for the controlled release of drugs inside patients. Their uses reach beyond medicine: they also feature in mobile air-conditioning systems and as industrial lubricants.
• Pronuclear transfer: this reproductive technique moves the nuclear material of a fertilised egg into a donor egg with healthy mitochondria, preventing mitochondrial disease in the child.
• ELISA: this is a standard test for detecting infections such as HIV. It is not a basic test for cancer.
• Rh blood group: about 85 percent of people are Rh-positive, carrying the Rh antigen on their red blood cells. The remaining minority are Rh-negative.

In India, health is largely a State responsibility. Under the country's decentralised system, the States are primarily in charge of organising health services. Historically the public system has leaned towards curative care, treating the sick, with less spent on preventive, promotive and rehabilitative care.

Several national schemes try to fill these gaps, especially for mothers, children and the poor:

• National Rural Health Mission (NRHM): it placed an ASHA, an Accredited Social Health Activist, in each village. The ASHA is a trained community health worker who escorts women for antenatal checkups, uses pregnancy test kits, and spreads information on nutrition and immunisation. She does not herself conduct deliveries.
• Janani Suraksha Yojana (JSY): a safe-motherhood scheme that aims to cut maternal and newborn deaths among poor women and to promote institutional delivery, that is, childbirth in a health facility. It is a Centrally Sponsored Scheme, funded by the Centre and run through the States, not a programme of the State health departments alone. It does not cover the care of sick infants up to one year of age.
• Pradhan Mantri Surakshit Matritva Abhiyan (PMSMA): it offers fixed-day antenatal care to pregnant women at government facilities, where private specialists may volunteer their services.
• Anaemia Mukt Bharat: it supplies iron and folic acid (not calcium), runs deworming and delayed cord-clamping campaigns, and tackles non-nutritional causes of anaemia such as malaria, blood disorders and fluorosis.
• Ayushman Bharat Digital Mission: it builds digital health records with portability across the country. Its adoption by hospitals is voluntary, not compulsory.
• Rashtriya Swasthya Bima Yojana (RSBY): a health insurance scheme launched by the Ministry of Labour and Employment, not the Health Ministry. A separate scheme, the Rajiv Gandhi Shilpi Swasthya Bima Yojana for handicraft artisans, was launched by the Ministry of Textiles.