Health is a state of complete physical and mental wellbeing. It is not just the absence of disease. A person can have no illness at all and still be in poor health, for example if they are very stressed or anxious.
Disease and health affect each other. Having a disease makes health worse. Poor health can also make disease more likely: a poor diet, high stress or a weak immune system makes it easier to catch an infection.
Physical and mental health are linked too. A long-term physical illness can cause anxiety or depression, and mental ill health can affect the body. Diet, stress and a person's life situation all influence health.