Hamburger, say, one that infects a number (6)

I believe the answer is:
german
'hamburger' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'one that infects a number' is the wordplay.
'one that infects' becomes 'germ' (I can't explain this - if you can you should give a lot more credence to this answer).
'number' becomes 'n' (n can represent any number).
'germ'+'a'+'n'='GERMAN'
'say' acts as a link.
Can you help me to learn more?
(Other definitions for german that I've seen before include "Manger (anag.)" , "European language" , "This national rang me up" , "Teutonic language" , "Helmut?" .)
