But Simon's wife's mother lay sick of a fever,.... "A great fever", Luke says, Luke 4:38; a very violent one, which threatened with death, and must be very dangerous to an old person; See Gill on Matthew 8:14,

And anon they tell him or her; for it seems, that not as soon as he came into the house, but some time after, when he had sat awhile, and rested himself after his fatigue in preaching; they acquainted him with her case, and beseeched him to look upon her, and restore her: this was done, either by Simon and Andrew, or by some others of their friends that were in the house; who having either seen, or heard of his dispossessing the unclean spirit, might rightly conclude he had power to remove a fever.

Lay sick of a fever (κατέκειτο πυρέσσουσα)

Κατά, prostrate. Mark adds, they tell him of her. Luke, they besought him for her. Mark, he came to her. Luke, he stood over her. Mark only, he took her by the hand and raised her up.

But Simon's wife's mother lay sick of a fever, and anon they tell him of her.

1:29-34 For notes on the healing of Peter's wife's mother, see Mt 8:14.

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.

1:29-39 Wherever Christ comes, he comes to do good. He cures, that we may minister to him, and to others who are his, and for his sake. Those kept from public ordinances by sickness or other real hinderances, may expect the Saviour's gracious presence; he will soothe their sorrows, and abate their pains. Observe how numerous the patients were. When others speed well with Christ, it should quicken us in seeking after him. Christ departed into a solitary place. Though he was in no danger of distraction, or of temptation to vain-glory, yet he retired. Those who have the most business in public, and of the best kind, must yet sometimes be alone with God.