What health means

1. Health is a state of complete physical, mental and social wellbeing, so it is never measured only by whether a person is free of illness.
2. Good physical health means the cells, tissues, organs and organ systems of the body are all working as they should.
3. Good mental health means a person can think clearly, manage their emotions and cope with the ordinary demands of daily life.
4. Good social wellbeing means having secure housing, enough food, safe water and supportive relationships with other people.
5. A disease is a condition in which part of an organism stops working normally, and the changes this produces are called symptoms.
6. A person can be carrying no pathogen at all and still be in poor health, for example someone living in cold damp housing on a diet that is short of protein and vitamins.
   1. The reverse also holds, because a person whose long term condition is well controlled by medicine can be in good general health.

Factors that affect health

1. Diet affects health because the body needs energy, protein, vitamins and mineral ions to build tissues and repair damage.
2. Stress affects health because long periods of it disturb sleep, appetite and mood, which makes existing conditions harder to control.
3. A person's life situation affects health, including their housing, income, working conditions and whether clean water and sanitation are available.
4. Access to medical care affects health because a disease that is diagnosed and treated early usually causes far less damage than one left untreated.
5. Regular physical activity keeps the heart, lungs and muscles working efficiently and helps hold body mass in a healthy range.

Communicable and non-communicable diseases

1. Every disease belongs to one of two groups, and the group depends on a single question: can it be passed to another organism?
2. A communicable disease is caused by a pathogen, a microorganism such as a virus, bacterium, fungus or protist, which reproduces inside or on the host.
   1. Because the pathogen itself can move between organisms, the disease can spread through a population.
3. Communicable diseases of humans include measles, influenza, cholera, tuberculosis and athlete's foot.
4. Plants suffer communicable diseases too, for example tobacco mosaic virus in tomato plants and crown gall disease in fruit trees.
5. A non-communicable disease is not caused by a pathogen, so nothing can move from a sick organism to a healthy one and it cannot be passed on.
6. Non-communicable diseases include coronary heart disease, type 2 diabetes, most cancers, arthritis and asthma.

Comparing the two types

1. A communicable disease usually appears quickly, often within days of the pathogen entering the body, because the pathogen reproduces rapidly.
2. A non-communicable disease usually develops slowly over years, so symptoms may appear long after the cause first began acting.
3. A communicable disease can often be treated by killing the pathogen, for example by using antibiotics against a bacterium.
4. A non-communicable disease cannot be cured by killing a pathogen, because there is no pathogen present to kill.
5. The number of cases of a communicable disease can rise sharply in weeks, whereas the number of cases of a non-communicable disease changes slowly as diets, activity levels and the age of a population change.

Deficiency diseases

1. A deficiency disease is one type of non-communicable disease, caused by a diet that is short of a particular nutrient or mineral ion.
2. A shortage of carbohydrate removes the body's main fuel for respiration, so it breaks down its own stored fat and then its muscle protein to release the energy it needs.
   1. The person loses body mass, tires easily and feels cold, because less energy is being released to keep the body warm and active.
   2. When the shortage of energy is severe and long lasting in a young child the result is marasmus, in which the fat stores and muscles are wasted away and the child is far below the expected body mass for their age.
3. A shortage of protein starves the body of the amino acids it needs to build new cells, repair damaged tissue and make enzymes and antibodies.
   1. Growth slows or stops in children, wounds heal slowly, and infections take hold more easily because fewer antibodies can be made.
   2. Severe protein shortage causes kwashiorkor, whose most recognisable sign is a swollen abdomen, caused by fluid leaking out of the blood and collecting in the tissues.
4. A shortage of lipids leaves the body without its densest energy store, without the insulating fat layer under the skin and without the raw material for cell membranes.
   1. The person loses heat quickly and cannot keep a steady body temperature in the cold, and the skin becomes dry and scaly.
   2. Vitamins A, D, E and K dissolve in fat rather than in water, so a diet with almost no lipid in it also leads to a shortage of those vitamins.
5. A shortage of dietary fibre matters even though fibre is not digested and releases no energy, because it is the bulk that gives the gut something to grip.
   1. Fibre holds water and keeps the contents of the intestine soft and bulky, so the muscles of the gut wall can squeeze them along by peristalsis.
   2. Without enough fibre the contents move slowly, more water is absorbed from them, and the result is constipation and a raised risk of bowel disease.
6. Too little vitamin C causes scurvy, in which the gums bleed and wounds heal slowly, because vitamin C is needed to make the protein collagen.
7. Too little vitamin D or calcium causes rickets in children, because the leg bones stay soft and bend under the weight of the body.
8. Too little iron causes anaemia, because iron is needed to make haemoglobin, so the blood carries less oxygen and the person tires quickly.
9. Every one of these conditions is corrected by returning the missing nutrient to the diet, which is further evidence that no pathogen is involved and that none of them can be passed on.