Infectious agents

Viruses, bacteria, and parasites that may cause cancer or increase the risk that cancer will form. Some viruses can disrupt signaling that normally keeps cell growth and proliferation in check. Some infections weaken the immune system, making the body less able to fight off other cancer-causing infections. And some viruses, bacteria, and parasites also cause chronic inflammation, which may lead to cancer. [From https://www.cancer.gov/about-cancer/causes-prevention/risk/infectious-agents]