Factors affecting human health can be divided into two categories:

I. Intrinsic or internal factors

II. Extrinsic or external factors

I. Intrinsic factors

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Disease-causing factors found within the human body are called intrinsic factors. They are:

1. Malfunctioning of body parts:

Many diseases are caused due to improper functioning of body organs. For example, cardiac failure (heart attack) is caused by improper functioning of heart and short-sightedness, long-sightedness and cataract are caused due to improper functioning of eye.

2. Hereditary diseases:

Diseases, which are passed on from generation to generation, are called hereditary diseases. Haemophilia, a rare disease in which the patient’s blood doesn’t clot easily and sickle cell anaemia, a disease in which the patient’s red blood cells become sickle-shaped due to the presence of defective haemoglobin are hereditary diseases.

3. Hormonal diseases:

Hormones control most of our body functions. A defect in the production or action of hormones can cause hormonal diseases. Diabetes is a disease caused due to the underproduction of insulin hormone by pancreas. Overproduction of growth hormone (GH) produced by pituitary leads to development of giants while its underproduction leads to development of dwarfs.

4. Allergy:

Some people are sensitive to dust, pollen, certain drugs and foodstuffs and develop rashes on the skin or begin to sneeze. This type of hypersensitivity to certain substances is called allergy and can be prevented by avoiding contact with the allergy causing substance.

II. Extrinsic factors:

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Disease causing factors coming from outside the human body are called extrinsic factors. They are:

1. Nutritional disorders:

Diseases caused by deficiency or excess of essential nutrients in diet are called nutritional disorders. Deficiency diseases like kwashiorkor, scurvy, rickets, goiter, anaemia, etc., are caused due to deficiency of essential nutrients in the body.

2. Infection by microorganisms:

Micro-organisms (bacteria, viruses, fungi and protozoa) enter the human body through contaminated food, water and air, through skin contact and insect-bites.

3. Habit forming substances:

Substances like alcohol, tobacco and narcotic drugs tend to make a person increasingly dependent on them. Hence, they are called habit-forming substances. Excessive intake of tobacco in the form of bidi, cigarette or cigar and direct chewing can lead to accumulation of tar in the nose, throat and lungs causing cough and irritation. Tobacco smokers also run a higher risk of heart attack.

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Alcohol is consumed in the form of wine, whiskey, beer, gin, etc. Alcohol affects the nervous system. It reduces the alertness of mind. It increases the time a person normally takes to respond or react to an emergency. (This time is called reaction time.) Excessive intake of alcohol can damage the liver.

Opium and heroin are narcotic drugs. They are highly addictive and it is highly difficult to give them up. Narcotic drugs damage the nervous system and adversely affect vision, hearing respiratory system and heart. Excessive intake of narcotics may lead to heath.

4. Environmental pollution:

Dust, pollen, carbon monoxide, carbon dioxide, sulphur dioxide and nitrogen oxides pollute the environment. They cause headache, cough, allergies, lung disorders and bronchitis. Pesticides and chemical wastes from industries can cause damage to human health.