Most people think 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.
Physical well-being means the body works properly and is free from illness or injury. Mental well-being means being able to cope with everyday stress and feeling content, without distress. Social well-being means having supportive relationships and being able to take part in the community.
All three parts matter. A person with no disease can still be unhealthy if they are constantly anxious or very lonely.