ותשחת [THE EARTH] WAS CORRUPT — It means lewdness and idolatry, as (Deuteronomy 4:16) פן תשחיתון “lest ye deal corruptly” (the following words show that this refers to idolatry) and as ‘כי השחית כל בשר וגו “for all flesh had corrupted etc.” (in next verse of this chapter where Rashi states that this has reference to lewdness) (Sanhedrin 57a).
The Rabbenu Bachaya explains the difference between a doctor where it says verapah yerapeh with a ‘peh’ heavy, whereas by Hashem it says rofecha with a ‘feh’ light. Hashem’s cure is light but a doctor’s
is hard. By a doctor there is a double expression whereas by Hashem a single one. A doctor leaves side effects requiring to go back to heal. I also heard that we should seek a second opinion which are by Hashem unnecessary.