The town is ripe for the witch trials because of the strict Puritan religion, which forbids nearly everything but hard work and church. To rebel against this strict code, the girls flee into the woods for their little ceremony. However, once they realize Parris has seen them dancing and frolicking, they know what harsh consequences await them once words gets out. So they feign mysterious illnesses. Because the Puritans were so strict and believed that Satan was ever present in their lives, waiting to snare them, they were susceptible to the girls' lies about witch craft. Once the accusations are leveled, the events compound.
Soon this becomes the perfect opportunity for the townspeople to take revenge on those they envied. So if one person coveted another person's land, they could spread rumors about that person being a witch and they would be seized by the court and stripped of all property and possessions. Then the accuser could purchase the land. Notice too that Giles states he has a source who claims Putnam persuaded his daughter to accuse people so he could get their land.
These are but a few of the reasons the town became so swept up in insanity of the witch trials.
Your question is somewhat vague. There are many events that occur in the story. I assume you are referring to the beginning of the story, of the illness that has befallen Betty Parris and the accusations that follow. The town is concerned with Betty's illness because it is so unusual and unexplained. The accusations of the girls challenge all that the townspeople believe is righteous and holy. To be told that their worst fears are coming true, that their children are being affected by the devil, touches the fear that no one is safe from sin. At a time when the community is controlled by religion, and when adherence to that religion controls a person's success, this situation makes everyone vulnerable.