This is the 195th most frequent Polish word.
zdrowia
The Polish word "zdrowia" means "health" (genitive singular form).
In this sentence, 'zdrowia' is used in its genitive case form as it represents an abstract concept of health being wished.
Życzę ci dużo zdrowia.
I wish you plenty of health.
The word 'zdrowia' is in its genitive case form indicating the state to which someone was restored.
Po chorobie w końcu powrócił do zdrowia.
After the illness, he finally returned to health.
Here, 'zdrowia' is in its locative case form, referring to something of primary importance to care for.