At the very beginning of World War II, all Polish Jews of Wyszków, including Lajchers' family, were expelled by the Nazis in one massive action of 4 September 1939.
The older 77 Jews, along with 8 Poles who were helping them, were locked in a barn and burned alive.
Later that month, another 65 Jews were shot; afterward the town was declared Judenfrei.
 The Lajchers relocated to Węgrów, which was already swelling with hundreds of expellees.
In the summer of 1940, Lajcher joined the local Jewish council and organized a hospital.
In February 1941 the ghetto was closed off from the outside and hunger set in amongst its inmates.
Lajcher wrote letters to the American Jewish Joint Distribution Committee, but in vain.

The extermination of Jews by semi-industrial means throughout the country began in early 1942 and continued until all Jewish ghettos in German-occupied Poland were liquidated.
The first Węgrów ghetto action began at dawn on 21 September and concluded on 22 September 1942, with up to 5,000 Jews expelled to Sokołów Podlaski after a wave of ad hoc executions.
A small ghetto was created in its place.
Following the liquidation of the small ghetto in Wegrów on 26–27 April 1943, during which his wife and 13-year-old son were murdered, Lajcher was brought to Treblinka in a Holocaust train on 1 May 1943.

Treblinka was built as part of the most deadly phase of the Final Solution, known as Aktion Reinhard, and operated between 23 July 1942 and 19 October 1943.
During this time, more than 800,000 Jews – men, women, and children – were murdered there, with other estimates exceeding 1,000,000 victims.

In Treblinka, Lajcher was put in charge of a small infirmary for the SS after the suicide of his predecessor, Dr. Julian Chorążycki (not to be mistaken with the "fake" infirmary called "lazaret" where the hands-on killing took place).
Asked by the Underground, according to Samuel Rajzman, he also agreed to take the leadership in their secret escape plan.
The Organizing Committee at Treblinka Totenlager included Zelomir Bloch (leadership), Rudolf Masaryk, Marceli Galewski, Samuel Rajzman, Dr. Irena Lewkowska (sick bay), Leon Haberman, and several others.
The timing became imperative after Chorążycki was ambushed by Kurt Franz and swallowed a deadly poison.
Lajcher launched the uprising on a hot summer day when a group of Germans and Ukrainians drove off to the Bug River for a swim.
At the very beginning of World War II, all Polish Jews of Wyszków, including Lajchers' family, were expelled by the Nazis in one massive action of 4 September 1939. The older 77 Jews, along with 8 Poles who were helping them, were locked in a barn and burned alive. Later that month, another 65 Jews were shot; afterward the town was declared Judenfrei.  The Lajchers relocated to Węgrów, which was already swelling with hundreds of expellees. In the summer of 1940, Lajcher joined the local Jewish council and organized a hospital. In February 1941 the ghetto was closed off from the outside and hunger set in amongst its inmates. Lajcher wrote letters to the American Jewish Joint Distribution Committee, but in vain. 
The extermination of Jews by semi-industrial means throughout the country began in early 1942 and continued until all Jewish ghettos in German-occupied Poland were liquidated. The first Węgrów ghetto action began at dawn on 21 September and concluded on 22 September 1942, with up to 5,000 Jews expelled to Sokołów Podlaski after a wave of ad hoc executions. A small ghetto was created in its place. Following the liquidation of the small ghetto in Wegrów on 26–27 April 1943, during which his wife and 13-year-old son were murdered, Lajcher was brought to Treblinka in a Holocaust train on 1 May 1943.
Treblinka was built as part of the most deadly phase of the Final Solution, known as Aktion Reinhard, and operated between 23 July 1942 and 19 October 1943. During this time, more than 800,000 Jews – men, women, and children – were murdered there, with other estimates exceeding 1,000,000 victims.
In Treblinka, Lajcher was put in charge of a small infirmary for the SS after the suicide of his predecessor, Dr. Julian Chorążycki (not to be mistaken with the "fake" infirmary called "lazaret" where the hands-on killing took place). Asked by the Underground, according to Samuel Rajzman, he also agreed to take the leadership in their secret escape plan. The Organizing Committee at Treblinka Totenlager included Zelomir Bloch (leadership), Rudolf Masaryk, Marceli Galewski, Samuel Rajzman, Dr. Irena Lewkowska (sick bay), Leon Haberman, and several others. The timing became imperative after Chorążycki was ambushed by Kurt Franz and swallowed a deadly poison. Lajcher launched the uprising on a hot summer day when a group of Germans and Ukrainians drove off to the Bug River for a swim.
Berek Lajcher