How does the setting change by the end of the novel?

The novel is set in Illinois in the 1970s, in the suburbs of Chicago. In the heart of the Midwest, the young characters are safe and secure before the virus that kills all adults. After the virus, the setting takes on a different aspect. The children are surrounded with the remnants of civilization. They need to find ways to work with the resources they have to survive. One of the resources the children have is a nearby lake. They can find water there, and they hold celebrations there, with bonfires. The lake provides both sustenance and a social environment for the children.