We are used to considering viruses as enemies, tending to forget the multitude of their other applications in biotechnology and biomedicine. For one thing, did you know that viruses could be weapons against cancer cells? In fact, that is precisely what oncolytic viruses are used for. Moreover, a review recently published on Nature Immunology explores their possible combination with other anti-neoplastic agents and especially with cancer immunotherapy. Viruses and immune system: two sworn enemies that somehow work “together” against tumour cells.