This is the 10271st most frequent Russian word.
больному
"Больному" means "to the sick person" or "to the patient" in the dative case.
In this sentence, 'больному' is used in the dative case to indicate the recipient of the action: the patient who is receiving the medicine.
Врач передал лекарства больному.
The doctor handed the medicine to the patient.
Here, 'больному' functions as an adjective in the dative case, modifying 'человеку' to specify that the person receiving help is sick.
Помощь была оказана больному человеку.
Assistance was given to the sick person.
In this example, 'больному' is used in the dative case to describe the animal that needs attention, emphasizing its condition as sick.