It is easy to think that being healthy just means not being ill. The World Health Organization (WHO) takes a wider view. It defines health as a state of complete physical, mental and social well-being, and not merely the absence of disease or infirmity. Infirmity means weakness or frailty of the body.
There are three parts to this. Physical well-being means the body works properly and has enough energy for daily life. Mental well-being means being able to cope with stress, think clearly and feel positive. Social well-being means having supportive relationships and being able to take part in the community.
A person can have no disease at all and still not be fully healthy. Someone who is constantly anxious lacks mental well-being, and someone who is isolated and has no friends or support lacks social well-being. Only a person who is well in all three ways is healthy by the WHO definition.