Access old information to discover source of disease (8)
I believe the answer is:
pathogen
'source of disease' is the definition.
(I've seen this before)
'access old information' is the wordplay.
'access' becomes 'path' (I can't explain this - if you can you should believe this answer much more).
'old' becomes 'o' (abbreviation).
'information' becomes 'gen' (gen can informally mean information).
'path'+'o'+'gen'='PATHOGEN'
'to discover' acts as a link.
Can you help me to learn more?
(Other definitions for pathogen that I've seen before include "Disease-causing organism" , "it may make one sick" , "Disease-causing agent" , "source of disease" , "Heptagon (anag.)" .)