<p>The connections between viruses and cancer have historically been studied in the context of viral oncogenesis. For decades, tumour virology has focused on oncogenic viruses such as hepatitis B virus, hepatitis C virus, human papillomavirus, Epstein–Barr virus, human T cell leukaemia virus type 1, Kaposi sarcoma-associated herpesvirus and Merkel cell polyomavirus, elucidating their oncogenic mechanisms, which include mutagenesis, chronic inflammation and immune evasion. However, the human virome is vast and complex, and this oncogenesis-centred view has overshadowed the possibility that certain viral exposures enhance antitumour immunity. Through millions of years of coevolution with animal hosts, the virome, consisting of diverse bacteriophages and eukaryotic viruses, including endogenous retroviruses, appear to have evolved strategies for coexistence that shape immune development and potentiate host surveillance pathways capable of recognizing and eliminating cancer cells. Non-oncogenic viruses can prime innate and adaptive immune responses, mimic tumour antigens and modulate the expression of immune checkpoints, as exemplified by the association of the enterovirus and rhinovirus CE1 epitope with protective liver cancer immunity. Moreover, endogenous retroviruses, naturally occurring oncolytic viruses and microbiome-associated phages may act as allies in cancer control. This Review explores the emerging evidence for viral anticancer immunity, its underlying mechanisms, and implications for a virome-guided framework for cancer prevention including new approaches to risk assessment, immune-based therapeutics and applications in low-resource settings.</p>