This is the 11693rd most frequent Russian word.
заболевают
"Заболевают" means "fall ill" or "get sick."
The word 'заболевают' (fall ill) here describes individuals beginning to suffer from sickness.
Когда холодно, люди часто заболевают.
When it is cold, people often fall ill.
In this sentence, 'заболевают' conveys the tendency for children with weak immunity to start experiencing illness.
Дети обычно быстро заболевают, если их иммунитет ослаблен.
Children usually quickly fall ill if their immunity is weakened.
Here, 'заболевают' indicates the onset of a specific illness, which is pneumonia in the context given.