Sforno on Genesis 12:17/ספורנו על בראשית י״ב:י״ז

1וינגע ה' את פרעה נגעים גדולים, only Pharaoh was struck with “great” afflictions,

2whereas his household suffered lesser afflictions. This was arranged by G’d so that the Egyptians, when they saw that only Sarai had remained completely immune to these afflictions, would conclude that the afflictions were on account of her and her fate.