Immediately after Germany invaded Poland in September 1939, most Jews, especially in the cities, were forced to live in ghettos. These ghettos were surrounded by barbed wire and were in fact concentration camps. Anyone who left the ghetto was shot dead. In October 1939, the first ghetto was set up in Piotków Trybunalski (a city south of Łódź). By the end of October 1939, Jews in Wloclawek were forced to wear the yellow star (jewish star), a measure which in 1941 was implemented throughout Europe. In the course of 1940 and 1941, increasingly more Wohngebiete der Juden (living areas for Jews) were created. The largest ghettos were in Warsaw (380,000 people) and Łódź (160,000 people). Jewish ghettos were also built in other European countries. People in the Polish ghettos lived under unspeakable conditions. In Warsaw, 30% of the population had to live in less than 2.5% of the city's area. The ghetto's inhabitants suffered extreme malnutrition because they depended on the Nazis for food. In the ghetto of Łódź 95% of the houses did not have sanitary facilities and running water. Hundreds of thousands of Jews died in the ghettos. In the ghettos, the nazis used the Jewish councils to communicate their orders. This administrative system, which made the Jews partially responsible for their own destruction, was also applied in other places in occupied Europe.