Exception
-er der
Usually 81.3% of the 2,315 common nouns ending in -er take der.
Fieber breaks this rule: it is das Fieber, not der Fieber.
This is only a tendency. Common exceptions are family words (die Mutter, die Tochter) and das Zimmer, das Fenster.
Other exceptions die Mutter, die Tochter, die Schwester, das Wasser