What causes low-grade fever in children after recovering from influenza B and fever subsiding for 3 days?

A child with influenza B who develops low-grade fever after three days of defervescence may experience this due to the following reasons: 1. The influenza B virus itself may cause inflammatory responses in sites such as the respiratory tract and gastrointestinal tract, leading to elevated body temperature. After the fever subsides, the illness may not have fully resolved, and the virus may still be replicating in the body, resulting in persistent low-grade fever. 2. Following a viral infection, a child's immune system may be somewhat suppressed, making them susceptible to secondary bacterial or viral infections. These new infections can trigger additional inflammatory responses, causing prolonged low-grade fever.