In "The Crucible" why has Reverend Parris sent for a doctor as the play begins?

At the beginning of act one, if you read the text closely, it soon becomes clear that Betty, Reverend Parris's daughter, is lying on the bed, very ill.  Reverend Parris is freaking out because he has no idea what is wrong with her--she is not acting normal.  She is just lying there, not speaking, not eating, not moving, nothing.  It is almost like she is in a coma of sorts.

Parris is also very worried, because the night before, he discovered his daughter and his niece Abigail (who lives with him) dancing in the woods with a bunch of other girls from the village.  But that's not all--as the scene proceeds, we learn that he thinks he saw someone naked running around, and, that there was a big kettle of boiling liquid, with a frog in it.  All of these things put together, make Reverend Parris very upset; he suspects some sort of conjuring of spirits, and other forbidden or wicked behavior by the girls.  So, when he discovered them there, he was understandably very upset.  His daughter, Betty, since that incident, has been acting strange.  He is worried that maybe she has been bewitched somehow, through the activities that were occurring.  He hopes not, because that would cast a shadow on his reputation as a minister, so, he sends for the doctor.  He hopes that the doctor can find some easily explained medical reason for Betty's strange behavior.

I hope that clears things up for you; good luck!