Immunotherapy may hold the key to defeating virally associated cancers

The link between viruses and cancer Suspicions about a possible role of viral infections in the development of cancer were fi rst aroused in the early 1900s. Th e seminal discovery is traced back to Peyton Rous, who showed that a malignant tumor growing in a chicken could be transferred to a healthy bird by injecting it with tumor extracts that contained no actual tumor cells.1 Th e infectious etiology of human cancer, however, remained controversial until many years later when the fi rst cancer-causing virus, Epstein-Barr virus (EBV), was identifi ed in cell cultures from patients with Burkitt lymphoma. Shortly afterward, the Rous sarcoma virus was unveiled as the oncogenic agent behind Rous’ observations.2 Seven viruses have now been linked to the development of cancers and are thought to be responsible for around 12% of all cancer cases worldwide. Th e burden is likely to increase as technological advancements make it easier to establish a causal link between viruses and cancer development.3 In addition to making these links, researchers have also made signifi cant headway in understanding how viruses cause cancer. Cancerous transformation of host cells occurs in only a minority of those who are infected with oncogenic viruses and often occurs in the setting of chronic infection. Viruses can mediate carcinogenesis by direct and/ or indirect mechanisms (Figure 1). Many of the