A non-communicable disease cannot be spread from one person to another, because no pathogen is involved. Many are caused by the interaction of several factors, including the genes a person inherits and their lifestyle. The lifestyle factors to know are exercise, diet, alcohol and smoking.
Smoking raises the risk of lung cancer, lung disease and cardiovascular disease. Too much alcohol can damage the liver and cause cirrhosis. A diet high in saturated fat and sugar, with little exercise, leads to obesity, which raises the risk of cardiovascular disease and of type 2 diabetes.
The incidence of a disease is the number of new cases in a population. To compare places with different numbers of people, it is given per 100 000 people: cases ÷ population × 100 000. For example, 12 cases of lung cancer in a town of 40 000 people is 12 ÷ 40 000 × 100 000 = 30 cases per 100 000 people. This can be done at local level (a town or area), national level (a country) and global level (the world). Rates of type 2 diabetes, for example, are rising in many countries as diets and activity levels change.
When a lifestyle factor and a disease occur together, there is a correlation. A correlation does not prove causation. Scientists study a large sample and control other variables before they decide that a factor causes a disease.