Showing Forest Ghost by Graham Masterton

Modern-day America. Fifteen Boy Scouts and their seven adult leaders are found to have committed suicide in the forest of a scout reservation. One of the dead boys is a friend of Sparky Wallace, whose father Jack runs a Polish restaurant in Chicago. Drawn into investigating the suicides, Jack discovers a connection with his own grandfather, who killed himself in the Kampinos Forest in Poland when he was fighting the Nazis in World War II.

Together, Jack and Sparky travel to Poland to unlock the terrifying mystery of what really makes people panic in the forest. But before they can do so, they have to experience panic for themselves, and reach the very brink of madness.