Crossword Genius

Rare infection of the lung? Pressure is controlled with drug (6)

Ross

I believe the answer is:

sparse

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'rare' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

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(Other definitions for sparse that I've seen before include "Infrequent" , "Thinly scattered, not dense" , "Thinly populated" , "Thinly-spread" , "rarely occurring" .)

I've seen this clue in The Hindu.
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