Influenza A virus (IAV) infection represents a global threat causing seasonal outbreaks and pandemics. Additionally, secondary bacterial infections, caused mainly by Streptococcus pneumoniae, are one of the main complications responsible for the enhanced morbidity and mortality associated with influenza infections. In this chapter, we discuss the state of the art of mathematical models of influenza infection. We present a reduced mathematical model representing influenza infection and the respective immune response. We introduce the most recent progress of mathematical modeling including secondary bacterial coinfections, immunosenescence, and responsiveness to vaccination.