Disease-causing organism – track oxygen data (8)
I believe the answer is:
pathogen
'disease-causing organism' is the definition.
(I've seen this before)
'track oxygen data' is the wordplay.
'track' becomes 'path' (synonyms).
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'data' becomes 'gen' (I've seen this in other clues).
'path'+'o'+'gen'='PATHOGEN'
(Other definitions for pathogen that I've seen before include "it may make one sick" , "It makes us sick" , "source of disease" , "cause of illness" , "Cause of disease" .)