John Gill Commentary Mark 1:30

"Now Simon`s wife`s mother lay sick of a fever; and straightway they tell him of her:" — Mark 1:30 (ASV)

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.