Some viral infections increase the risk of developing cancer. Bacteria-infections may also raise the risk of certain cancers. The most widely studied family of cancer-causing viruses are papillomaviruses (HPV, human papilloma virus), some of which contribute to chronic inflammation and eventually Cervical Cancer. In addition, papillomaviruses can cause other cancers, such as pharyngeal cancer. Long-term follow-up studies are underway to determine to what degree HPV vaccines often prevent cancer.

The Helicobacter pylori bacterium raises the risk of gastric cancer. Chronic inflammation of the liver (infections with hepatitis B and C viruses) is associated with increased risk of Liver Cancer. A vaccination program for hepatitis B virus (HBV), initiated in Taiwan in 1984, led first to a decrease in HBV prevalence, and subsequently to a substantial reduction in the incidence of Liver Cancer in vaccinated age groups.