Viruses are only able to reproduce by entering a living cell and manipulating the cell’s machinery to create more viruses. During the viral replication process, certain virus’s DNA or RNA affects the host cell’s genes in ways that may cause it to become cancerous. These viruses are known as oncogenic viruses, meaning viruses that cause or give rise to tumors. An estimated 15% of all human cancers worldwide are already linked to such viruses and this percentage may grow in the future.
As outlined below, multiple viruses from very different families are already known to be connected with cancer in humans. Studying theses cancer causing viruses and discovering new ones, may lead to vaccines and other approaches that prevent or treat certain human cancers in the future. Although it may seem appropriate to lump all cancer causing viruses into one group it would not be accurate. Each virus is a unique type that together represents a variety of virus families with different genomes and replication cycles.
It is important to note that the process from the initial viral infection to tumor formation is slow and inefficient; only a small percentage of viral infections progress to cancer years or decades after the initial infection. Other factors may increase the chance of cancer including immune system complications, cell mutations, exposure to cancer causing agents and hereditary susceptibility. The viruses now known or suspected of being linked to cancer in humans include human papilloma viruses, the Epstein-Barr virus, the hepatitis B and C viruses, the human herpes virus 8 and the human T-lymphotrophic virus-1.
Human papilloma viruses (HPVs)
There are over 100 different types of human papilloma viruses that can cause warts on the skin, mouth, genital organs and larynx. They are spread by human contact and are a common sexually transmitted disease. Currently there are no effective treatments for HPV other than the removal or destruction of infected cells. Over time, the body’s immune system usually controls and destroys the virus.
Specific human papilloma viruses are known to cause cervical cancer, which is the second most common cancer among women worldwide. In the United States, cervical cancer has decreased because of the availability of the Pap test. The test is performed to check for pre-cancerous cells of the cervix that could be caused by an HPV infection. If abnormal cells are seen, they can be removed to keep cancer from developing.
HPV is estimated to be the cause of 5% of cancers worldwide according to the National Cancer Institute. An oral HPV infection may also cause cancers of the oropharynx (the middle part of the throat including the soft palate, the base of the tongue and the tonsils).
Two vaccines, Gardasil and Cervarix, are available to be used against the types of HPV that cause cancer. The vaccines have been shown to help protect against infection from the two main cancer causing HPV types. Further research is currently being conducted to study these vaccines and others like them.
Epstein-Barr virus (EBV)
The Epstein-Bar virus is a type of herpes virus and that is best known for causing infectious mononucleosis, better known as “mono” or the “kissing disease”. This virus is very common, and it is estimated that about 95% of the US population is infected with the virus, though not everyone shows any symptoms. The virus can be spread person to person by kissing, coughing, sneezing or sharing a drink or eating utensils. The virus remains with the person throughout life by infecting epithelial cells and a type of white blood cells known as B lymphocytes or B cells. However, after a few weeks of initial infection, most people show no symptoms.
Infection of the Epstein-Barr virus causes about 15% of stomach cancers, most nasopharyngeal cancers (cancer of the nasopharynx, the region behind the nose and above the back of the throat), and certain types of lymphomas known as Burkitt or Hodgkin lymphomas.
Hepatitis B virus (HBV) and hepatitis C virus (HCV)
HBV and HCV are two types of viruses that cause viral hepatitis, a type of liver infection. Hepatitis A can also cause viral hepatitis but HBV and HCV are known to cause long term infections that increase chances of liver cancer. Worldwide, most liver cancers are caused by HBV or HCV.
Both HBV and HCV are spread most often spread through unprotected sex, sharing needles or childbirth. Hepatitis B virus, is known to cause flu like symptoms and jaundice, or yellowing of the eyes and skin. Most people with HBV infection recover completely and are not chronic carriers of the virus. Hepatitis C infection may not cause any symptoms and is more likely to be chronic, which can lead to liver damage or even cancer.
Antiviral drugs are available to treat people with Hepatitis B and C. There is also a preventative but not therapeutic vaccine available for Hepatitis B, which is most commonly given to people who are at risk of contracting the virus, such as health care workers. There is no vaccine available for the Hepatitis C virus.
Human herpes virus 8 (HHV-8)
The human herpes virus 8 is also known as Kaposi sarcoma(KS)-associated herpes virus (KSHV) and has been found in rare slow growing cancerous tumors that appear as reddish-purple or blue-brown underneath the skin known as KS. This virus is transmitted sexually and is present in about 10% of the US population, but the HHV-8 virus does not appear to cause disease in most healthy individuals. People who develop KS usually have a weakened immune system from the human immunodeficiency virus (HIV) or are taking immune suppression medication. Like other herpes viruses that cause cold sores or chicken pox, HHV-8 infections never go away. Researchers are still trying to determine how HHV-8 contributes to the development of Kaposi sarcoma
Human T-lymphotrophic virus-1 (HTLV-1)
HTLV-1 is a type of retrovirus that uses RNA (instead of DNA) for its genetic code. During the replication process, the virus uses an enzyme called reverse transcriptase, which allows the virus to change its RNA genes into DNA. This allows the virus genes to become integrated into the genes of the host cell and can cause a mutation in the host cell genes that controls how the cell divides. This change can sometimes lead to cancer. HTLV-1 has been linked to a type of lymphocytic leukemia and non-Hodgkin lymphoma called adult T-cell leukemia/lymphoma (ATL). It is most common in populations of southern Japan, Caribbean, Central Africa, and some areas in the United States.
Human T-lymphotrophic virus-1 is spread by unprotected sex, needle sharing, pregnancy, childbirth or breastfeeding. Once infected, a person may live 20 years or more with no symptoms. There is about a 5% chance that the infection will lead to cancer.