In summary, the combination of SEIRA spectroscopy and tBLMs provides a new tool to monitor key steps of IAV entry. While it lacks single-particle (i.e., single virion) resolution offered by fluorescence assays, SEIRA operates label-free avoiding the need for exogenous dyes. Furthermore, the approach benefits from the intrinsic molecular structuresensitivity of IR spectroscopy and the nanoscale surface-sensitivity of SEIRA. Notably, SEIRA enables the detection of virus-membrane interactions while the viral proteins remain embedded in intact virions, eliminating the need for protein isolation as in standard IR spectroscopic studies. As such, we detect specific spectral features of IAV's HA glycoproteins during receptor binding at host membrane model and differentiate between viral and isotope-edited host lipids during virus-membrane fusion. Therefore, this approach not only complements available viral activity assays, but also our previous SEIRA study of the action and inhibition of the IAV M2 channel within a viral membrane mimic. As such, SEIRA spectroscopy provides a versatile method to monitor key steps in viral infection from both perspectives, the viral membrane and the host cell membrane to investigate viral mechanisms and new strategies towards antiviral inhibition.