We take an unconventional approach to study how cells work, we use viruses! After millions of years of evolution, these miniaturised 'trojan horses' have learned all the 'PIN codes' and 'passwords' to access cellular resources and, at the same time, escape the defence mechanisms of our immune system. By following the infection path of selected viruses, from their arrival to the cell surface and intracellular trafficking to their amplification and spreading, we focus on the complex dynamics and regulation of macromolecular assemblies such as membrane vesicles, organelles and ribonucleoprotein complexes. Using an arsenal of interdisciplinary approaches and state-of-the-art imaging techniques, this journey takes us through virtually all aspects of cell biology and virology. Our goal is to identify the 'Achille's heel' of viral infections and develop innovative antiviral strategies.