Microbe spread a nameless disease (6,7)
I believe the answer is:
german measles
'disease' is the definition.
(German measles is an example)
'microbe spread a nameless' is the wordplay.
'microbe' becomes 'germ' ('germ' can be a synonym of 'microbe').
'spread' indicates anagramming the letters.
'a'+'nameless'='anameless'
'anameless' with letters rearranged gives 'anmeasles'.
'germ'+'anmeasles'='GERMAN MEASLES'
(Other definitions for german measles that I've seen before include "complaint" , "Rubella" , "viral illness" .)