This is the 362nd most frequent Russian word.
болезни
Diseases.
Here, 'болезни' (illnesses) is used to refer generally to diseases and how they propagate under certain conditions.
Болезни быстро распространяются в холодное время года.
Illnesses spread quickly in cold seasons.
In this context, 'болезней' is in the genitive plural form, showing preventive measures against diseases.
Для предотвращения болезней важно соблюдать личную гигиену.
To prevent illnesses, it is important to observe personal hygiene.
Here, 'болезней' is also in the genitive plural, emphasizing actions directed toward avoiding diseases.