How does the character Mary Warren create mass hysteria in The Crucible?

Mary Warren is a character whose actions are part of very pivotal plot moments in the play. She works as a servant to John and Elizabeth Proctor, a replacement for Abigail Williams who was relieved of her duties after she had an affair with John. Because Abigail is in love with John, and because she is devious and selfish, she sees the witchcraft accusations as an opportunity to remove Elizabeth from the picture, and so decides to accuse her. Mary becomes a pawn in Abigail's game.

Abigail sees Mary making a poppet in court one day, and asks Mary to give it to Elizabeth (the poppet is a small cloth doll, believed by some to be used for casting spells on a person it represents). Later, Abigail is witnessed at the Reverend Parris' home screaming out in pain, and reveals she has a needle sticking out of her belly. Abigail had apparently also asked Mary to leave the needle in the doll. The poppet and needle are discovered by Ezekiel Cheever, when he comes to the Proctor household to serve a warrant on Elizabeth, who has been accused by Abigail, and the doll is seen as evidence of Elizabeth's attempts to hurt Abigail, and proof that she is a witch. This plot point is important because it places John Proctor, a respected man but one known to be dismissive of the witchcraft accusations, squarely in the center of the hysteria. Elizabeth is arrested and taken away; John Proctor angrily demands that Mary come to court and tell the truth.

The courtroom scene that follows is a prime example of the mass hysteria that allows the situation to escalate, wherein many people are accused and some subsequently hanged. When Mary Warren attempts to tell the truth, Abigail pretends to see a bird on the rafters, as if a spirit has flown into it. She also starts repeating everything Mary says, and the other girls do the same. This childish game is seen by those present in court as being somehow caused by Mary Warren, and the girls' play-acting is so effective that the court believes witchcraft must be the reason for their bizarre actions. Mary breaks down emotionally and decides she wants to side with Abigail once again; the accusations fly, fast and furious, and the Reverend Hale departs in frustration. This climactic scene depicts the way that mass hysteria (defined as "a condition affecting a group of persons, characterized by excitement or anxiety, irrational behavior or beliefs, or inexplicable symptoms of illness") can sway rational people into condemning their own neighbors.