Jesus was invited to Simon's house after the synagogue service. At that time, Simon (later to be called Peter) had not yet been called to be a disciple (Luke 5:1-11). The extended family was there including Simon's wife's mother. She may have been old but she was certainly very unwell in bed, running a high fever. In those days, without any effective medicines, fever might lead to death. So the family were concerned and asked Jesus for help. In the same way that He addressed the demons, He commanded the fever to leave her; and it did.