Betty is the daughter of the Reverend Parris, spiritual leader to the Salemite congregation and Ruth is daughter of the wealthy and embittered Thomas Putnam and his equally resentful wife, Ruth.
Betty and Ruth had been discovered dancing in the woods with other girls from the village by the Reverend. When he confronted them during this activity, his daughter fell into a coma that she could not be revived from. Whilst Betty was comatose, Ruth, although displaying similar symptoms, was able to perform certain activities, as Mrs Putnam explains:
She ails as she must - she never waked this morning, but her eyes open and she walks, and hears naught, sees naught, and cannot eat. Her soul is taken, surely.
The only rational explanation for the girls' sudden condition is that they were overwhelmed and surprised by Reverend Parris' sudden appearance whilst they were performing a highly illegal and suspicious ritual. They were in the woods summoning spirits under the leadership of the Reverend's slave from Barbados, Tituba.
The girls were raised in an extremely restrictive society, a theocracy, where the Bible was both religion and law and respect for and adherence to The Book permeated through every aspect of society. It was a society where even the slightest infringement, such as not knowing all your commandments or working during the Sabbath, was enough to damn one. Furthermore, it was inculcated into the girls that Satan and his minions were a constant threat and were ready to pounce on whomever showed the slightest weakness. Those who strayed from the part of goodness and reverence, were sure to become the Devil's disciples.
When they were discovered, the girls were obviously horrified and believed, in that instant, that they would face severe sanction or punishment. They were obviously traumatized, since their actions in the woods resembled devil worship. The shock of their discovery could have been so severe that it caused them to actually faint, an uncontrollable response. However, when they realized that their actions would result in severe consequences, they played up and acted as if they were really ill.
Their only hope to avoid what they obviously knew would be a severe punishment along with the shame and suspicion that they would bring upon their households, was to blame supernatural and uncontrollable forces for their activities in the woods. It is for this reason that the girls continued to feign illness, which set in motion accusations of witchcraft and the series of tragic events that were to transpire later.