This is the 4353rd most frequent Russian word.
больных
"Больных" means "sick people" or "patients" in English.
The word "больных" acts as a noun, indicating the individuals who are sick.
Чтобы избежать инфекции, нужно изолировать больных.
To avoid infection, it is necessary to isolate the sick ones.
In this case, "больных" is an adjective in plural form agreeing with "животных," describing animals that are sick.
Больных животных лечение не всегда возможно.
Treating sick animals is not always possible.
Here, "больных" is used as a noun in plural form referring to sick people or patients.