Some of the deadliest and most frequent malignant tumours worldwide are caused by microbes. It has been estimated that viruses contribute to approximately 15–20% of all malignancies in humans. Tumour viruses are not only a global health problem, they have also been instrumental for seminal discoveries in fundamental cancer research and helped to shape the current concepts of multistep tumourigenesis and malignant progression. This article will summarize the basic principles of viral oncogenesis followed by examples of known and emerging human tumour viruses.

Key concepts:

• Oncogenic viruses are a major cause of cancer-related death worldwide.
  
• Oncogenic viruses have evolved to target key regulatory pathways of infected host cells through different mechanisms including insertional mutagenesis, gene piracy and specialized viral proteins, which target host cellular proteins (oncoproteins).
  
• The most common host cell signalling pathways targeted by tumour viruses are the retinoblastoma (pRB) and p53 tumour suppressor pathways.
  
• Tumour viruses frequently subvert host cell control of the cell division cycle, apoptosis, genome integrity and immune recognition.
  
• Human tumour viruses include DNA viruses (HPV, EBV, KSHV and HBV) and RNA viruses (HTLV-I and HCV).
  
• The most recently discovered emerging human tumour virus is Merkel cell polyomavirus (MCPyV).