Health is the state of physical and mental well-being. It is not just the absence of disease. Diseases are major causes of ill health, and they can be communicable (caused by pathogens and passed between people) or non-communicable (not passed between people). Other factors, including diet, stress and life situations, can have a big effect on both physical and mental health.
Different types of disease can interact. If the immune system has a defect, a person is more likely to suffer from infectious diseases. Viruses living in cells can be the trigger for cancers. Immune reactions that were 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 a disease occurs in a population. This is the incidence of the disease. Data can be shown in a frequency table, a bar chart or a histogram, and you may be asked to change numbers into a graph or read numbers from a graph. A scatter diagram plots two variables against each other, and a trend on it shows a correlation. A correlation does not on its own prove that one factor causes the other.
It is usually not possible to test everyone, so scientists use a sample. A sample should be large enough and chosen to represent the whole population, otherwise the results may be unreliable.