פירוש על שמואל א 5:9

And they were afflicted internally.וַיִשָֹּתְרוּ is identical with וַיִסָּתְרוּ for it is listed in the Masora among those words written with a 'שֹ' and explained as though it were written with a 'ס' meaning "there was a plague in the hidden parts of the body."²Or it may mean that the plague occurred at night when it was dark and they were unable to see the שְׁרָצִים, i.e., the plague was hidden from them.—Da’as Sofrim