The sin of Achan made the whole camp of Israel unclean. He took some items from Jericho when the people were very expressly commanded by Joshua not to do so (Joshua 6:18–19). In doing this, Achan brought God’s wrath upon the whole Israelite community.
The responsibility of Achan’s sin is extended to the whole nation in an example of corporate solidarity. Corporate solidarity involves the whole group being treated as a unity with the individual and the group being merged together (e.g., Joshua 7:11, Joshua 7:20).1
1 But the people of Israel broke faith in regard to the devoted things, for Achan the son of Carmi, son of Zabdi, son of Zerah, of the tribe of Judah, took some of the devoted things. And the anger of the LORD burned against the people of Israel.