This is the 10266th most frequent Russian word.
болезнях
"Illnesses" or "diseases."
The word 'болезнях' here is used to indicate 'diseases' in the prepositional case, referring to the topic of the conversation.
Он говорил о болезнях, которые передаются через контакт.
He spoke about diseases that are transmitted through contact.
This usage involves 'болезнях' showing the prepositional case associated with the word 'болезни', indicating rare diseases as a subject of description.
В книге описывается информация о редких болезнях.
The book describes information about rare diseases.
In this sentence, 'болезнях' appears in the prepositional case, relating to the theme of diseases as discussed subjects.