The Crucible explores how mass hysteria can devastate a community. Hysteria is most clearly seen in the villagers' irrational acceptance of the girls’ fabricated claims of witchcraft. One specific example of hysteria occurs in Act III when the girls, led by Abigail, accuse Mary Warren of witchcraft to prevent her from testifying against them. Though multiple people (including Mary) have claimed that the witchcraft accusations are false, the court refuses to be swayed.
Hysteria was a major factor in the many accusations of witchcraft that occurred throughout The Crucible. It helps to understand what hysteria is--an overwhelming fear and excitement that overrides all logic, and is often enhanced and intensified by the presence of others who are acting out on that fear.
Every time the girls in the play accuse someone of being a witch in court, hysteria played a role. One girl would pretend to get cold, or see a spirit, or to be attacked by a spirit, and would cry out in fear and pain; the other girls, seeing her do that, caught the emotion like a contagious disease (which is how hysteria works), and would imagine they felt or saw the same things, or at least would react to the fear in the room. Mary Warren herself, in speaking to the judges, explained how it all happened:
"I--I heard the other girls screaming, and you, Your Honor, you seemed to believe them, and I--It were only sport in the beginning sir, but then the whole world cried spirits, spirits, and I--I promise you, Mr. Danforth, I only thought I saw them but I did not."
So, for a specific example of hysteria, you can look to the end of Act One. Here, Tituba starts naming people who might be witches, and is praised for it. So, Abby figures this is her way out of getting in trouble, and starts naming names. By the end of the act, all of the girls have caught on and are hysterically crying out names.
Even better, the girls turn on Mary Warren in Act Three, pretending she is a little bird come to tear their eyes out. Abigail leads the charge, and all of the other girls follow. Pretty soon the emotion is so intense that Miller writes in the stage directions,
"She and all the girls run to one wall, shielding their eyes. And now, as though cornered, they let out a gigantic scream, and Mary, as though infected, opens her mouth and screams with them."
Note how Miller describes Mary's hysteria as an infection received from the other girls--that, right there, is hysteria. So, Mary joins them, and eventually accuses Proctor of bewitching her, and the courts, once again, are ruled by hysterics instead of logic.
Here is a video about hysteria in The Crucible: