Human Health and Disease

Health is not just the absence of disease — it is a state of complete physical, mental and social well-being. Good health lets a person work efficiently and enjoy life. Disease is any condition that disturbs the normal working of the body or mind.

Diseases are broadly of two kinds: infectious (communicable) diseases, caused by germs (pathogens) and spread from one person to another, and non-infectious diseases, which are not spread by germs (like cancer or genetic disorders).

Infectious diseases are caused by different kinds of pathogens:

• Bacteria — e.g. typhoid, tuberculosis (TB), cholera, pneumonia.
• Viruses — e.g. the common cold, influenza, dengue, chikungunya, AIDS.
• Protozoa — e.g. malaria (caused by Plasmodium, spread by the female Anopheles mosquito) and amoebiasis.
• Fungi — e.g. ringworm and other skin diseases.
• Worms (helminths) — e.g. Ascaris (roundworm) and filaria.

Pathogens spread through air, water, food, contact, or via vectors (carriers) like mosquitoes and houseflies. Good hygiene (clean water, proper sanitation, washing hands, controlling mosquitoes) and vaccination are the main ways to prevent infectious diseases.

The body has a built-in defence system — the immune system — that protects it against pathogens. Immunity is the ability of the body to resist or fight infection. There are two broad types:

• Innate (natural) immunity — present from birth; it gives general, non-specific defence. It includes barriers like the skin, mucus and stomach acid, and cells like white blood cells that engulf germs.
• Acquired (adaptive) immunity — developed during life after exposure to a particular pathogen; it is specific and has memory, so the body responds faster on a second exposure.

Acquired immunity works mainly through antibodies — special proteins made by white blood cells (lymphocytes) that recognise and bind to a particular germ (the antigen) and help destroy it. Because of memory cells, once you have fought off a germ you are often protected against it in future.

Vaccination (immunisation) uses this principle. A vaccine contains a weakened or killed germ (or part of it) that cannot cause disease but does trigger the immune system to make antibodies and memory cells. So if the real germ attacks later, the body destroys it quickly — the person is immune. Vaccines have controlled or wiped out many deadly diseases (for example, smallpox has been eradicated and polio nearly so). Giving ready-made antibodies (as in an anti-snake-venom injection) is passive immunity, which acts at once but does not last.

Some serious health problems deserve special mention.

AIDS (Acquired Immuno Deficiency Syndrome) is caused by the HIV virus, which attacks and weakens the immune system (especially certain white blood cells). As immunity collapses, the person falls prey to many infections. HIV spreads through unprotected sex, infected blood or shared needles, and from an infected mother to her baby — not by casual contact like shaking hands or sharing food. There is no complete cure yet, so prevention (safe practices, screened blood, clean needles, awareness) is vital.

Cancer is a non-infectious disease in which some cells lose control over their division and multiply uncontrollably, forming a mass called a tumour. A malignant tumour can spread to other parts of the body (a process called metastasis), which makes cancer dangerous. Cancer can be caused by certain chemicals (carcinogens, such as those in tobacco smoke), radiation and some viruses. Tobacco use is a major preventable cause. Early detection (screening) and treatments like surgery, radiation and chemotherapy can save lives.

Drug and alcohol abuse means the harmful, non-medical use of drugs and alcohol. Commonly abused substances include opioids (like heroin), cannabis, cocaine and alcohol; many are addictive, meaning the body craves them. Abuse damages health, behaviour, studies, family and society. Warning signs include falling performance, mood swings and withdrawal from friends and family. Prevention and help include avoiding peer pressure, seeking support from parents and teachers, counselling and rehabilitation. Saying "no" to drugs and leading a healthy, active life is the best protection.