Health is the state of physical and mental well-being. It means more than just not being ill. Diseases are major causes of ill health. Communicable diseases are caused by pathogens and can spread from one person to another. Non-communicable diseases, such as coronary heart disease and cancer, cannot be passed on. Other factors, including diet, stress and life situations, can also have a big effect on both physical and mental health.
Different types of disease can interact. A defect in the immune system makes a person more likely to suffer from infectious diseases. Viruses living in cells can be the trigger for cancers. An immune reaction first caused by a pathogen can trigger allergies, such as skin rashes and asthma. Severe physical ill health can lead to depression and other mental illness.
Scientists study how often diseases happen. The incidence of a disease is the number of new cases in a population in a set time. For example, 60 new cases in a town of 20 000 people in a year is 3 cases per 1000 people. Data can be shown in frequency tables, bar charts and histograms, and students must be able to change information between a table and a graph. A scatter diagram plots two variables against each other, and a clear pattern shows a correlation. A correlation does not prove that one variable causes the other.
It is not possible to test everybody, so studies use a sample of the population. A larger sample that is chosen at random is more likely to represent the whole population and gives more reliable conclusions.