Chronic stress that produces inflammation and anxiety in mice appears to prime their immune systems for a prolonged fight, causing the animals to have an excessive reaction to a single acute stressor weeks later, new research suggests. After the mice recovered from the effects of chronic stress, a single stressful event 24 days later quickly returned them to a chronically stressed state in biological and behavioral terms. Mice that had not experienced the chronic stress were unaffected by the single acute stressor.