healthy
The German word 'gesund' is an adjective used to describe a state of being healthy or in good health. It can be used to describe both people and things. For example, one might say 'Ich bin gesund' to express that they are in good health, or 'Das Essen ist gesund' to say that the food is healthy.
He lives healthily.
This statement is about someone's lifestyle, indicating that the person lives in a way that promotes his health. This could mean a balanced diet, regular exercise, and so on.
Stay healthy!
This is an imperative form of wishing someone to remain in a good health. It can be used in both formal and informal context.
I feel very healthy.
This is a statement someone might make about their own health, possibly in response to a query about how they are feeling. It means self-perception about one's health status.