This is the 2880th most frequent Russian word.
больной
"Больной" - sick; ill; patient (depending on context).
The word "больным" is used as an adjective here to describe someone's state of being unwell.
Он кажется больным, может быть, ему нужна помощь врача.
He seems ill, maybe he needs a doctor's help.
The word "больной" is used as a noun here to refer to a person who is in a hospital or requires care.
В палате был только один больной.
In the ward, there was only one patient.
The word "больна" is used metaphorically, meaning deeply affected or troubled by a matter.