Jamieson, Fausset and Brown on Mark 1:30

30. But Simon's wife's mother lay sick of a fever--Luke, as was natural in "the beloved physician" (Col 4:14), describes it professionally; calling it a "great fever," and thus distinguishing it from that lighter kind which the Greek physicians were wont to call "small fevers," as Galen, quoted by Wetstein, tells us.

and anon--immediately.

they tell him of her--naturally hoping that His compassion and power towards one of His own disciples would not be less signally displayed than towards the demonized stranger in the synagogue.