Reason for illness, oddly, lies in Soho obsession (10)

I believe the answer is:

leishmania

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'reason for illness' is the definition.
The definition suggests a singular noun which matches the answer.

'oddly lies in soho obsession' is the wordplay.
'oddly' indicates alternate letters (letters in odd-numbered positions).
'obsession' becomes 'mania' (I've seen this in another clue).
'lies'+'in'+'soho'='liesinsoho'
The alternate letters of 'liesinsoho' are 'leish'.
'leish'+'mania'='LEISHMANIA'

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