Mrs. Grose relates that both Quint and Miss Jesse are infamous, apparently for the open sexual relationship they have. At the end of this chapter, what is suggested bout Miss Jessel’s reason for leaving and her death

The fact that she was a governess and that put her in a higher social class than Quint made her behavior "infamous" because her relationship with Quint was completely inappropriate in this time period. The new governess believes that the two who have been infamous are now, supposedly, coming back to corrupt the children and cause great disruption.