Health is a state of complete physical, mental and social wellbeing. It is not just the absence of disease. A person can have no illness at all and still be in poor health if they are very anxious or isolated from other people.
A disease is a condition in which part of the body or mind does not work as it should. Disease can harm a person's health, but many other things affect health too, including diet, stress and a person's life situation, such as money worries or poor housing.
Health and disease affect each other. A long-term physical disease can make a person feel low and lead to poor mental health. In the other direction, high stress or a poor diet can weaken the body and make a person more likely to catch an infection.