Why is the Reverend Samuel Parris so distraught at the beginning of Act 1?

Rev. Samuel Parris is praying and weeping at his sick daughter's bedside. The doctor cannot find any reason for her illness and has no treatment for her condition. He sends word to Parris that he believes Betty's illness is "un-natural", something Parris does not wish to hear, particularly after finding his daughter, niece, and a number of other girls dancing in the forest like heathens.