This is the 11692nd most frequent Russian word.
заболевании
Illness or disease.
In this sentence, 'заболевании' (illness) is used in a general context referring to the state of being unwell or sick.
При заболевании важно обратиться к врачу.
During an illness, it is important to consult a doctor.
Here, 'заболевании' (an illness) is used to specify a medical condition that might have consequences if not treated properly.
Заболевании может привести к осложнениям.
An illness can lead to complications.
In this usage, 'заболевании' (an illness) introduces a specific health issue for discussion.