This answer can be found rather quickly in the first part of the play itself. Read the opening scene of Act One; in this scene, Betty is lying "sick" in bed, and her father, Reverend Parris, is alarmed and concerned at her behavior. Miller writes that she "cannot move herself" and is lying there inert, not responding to anything or anyone. Parris is suspicious of this behavior, considering he discovered her and other girls of the village "dancing like heathen in the forest." Dancing, in Salem, is forbidden, and he saw other things there too--Tituba chanting, a pot of boiling stew, a frog in the pot, and possibly, one of the girls running around naked. So, he was already alarmed at all of that behavior, and then, Betty starts acting strange. He sends for the doctor, hoping that he will be able to determine what is wrong. However, the doctor "cannot discover no medicine for it in his books." In other words, he has no idea what is wrong with Betty, or how to help her. The doctor then advises that Parris "look to unnatural things" as the source of her illness. This means that the doctor is recommending that Parris turn to the possibility of spells, witchcraft, or some other non-medical-related cause. Parris doesn't want to; having witchcraft labelled in his house would ruin his reputation as a preacher and cause unrest in the congregation.
I hope that those thoughts helped a bit; good luck!