no one else knows this secret, after professor lupin quit his job as a DADA professor, no one knew that he accidentally infected someone else, which was Snape, and it happened the moment he stood between the transformed lupin and the trio trying to protect them but got bitten in the process

Next year. The assistant was on her way to deliver some herbs to professor Snape, saying that it's so urgent for him for some potion making. She nervously made her way to the dungeons, as the light of the full moon illuminating her way to his office