I think I might have a bit of an idea of what it could be:

-spoilers ahead-


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In the promos/sneak peeks for next weeks episode, we see Sasha is ill and she goes to the cellblock where Dr. S is. I have a feeling Dr. S is dead, and Hershel will find out, possibly from Sasha or Tyreese when he visits his sister. Hershel will choose to go in there and take care of the ill, and try well, making them better. Putting his own life at risk, hence Maggie's "I cant let you do  this" and Beth's crying, which looks as if in the shot after Hershel is knelt talking to her.

What do you guys think?