The most common and most likely answer to this crossword clue is the 9 letter word CHLAMYDIA
I've seen this crossword clue in Independent
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The straight part
We know that "Infection" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case infection = chlamydia.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CHLAMYDIA mean?
CHLAMYDIA noun- Any of several common, often asymptomatic, sexually transmitted diseases caused by the microorganism Chlamydia trachomatis.
- Any of various coccoid microorganisms of the genus Chlamydia that are pathogenic to humans and other animals.