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Since the beginning of the 20th century, we have known that
some infections play a role in cancer in animals. But only recently has
infection with certain viruses, bacteria, and parasites been recognized
as a risk factor for several types of cancer in humans.
Worldwide, infections are linked to about 15% to 20% of
cancers. In the United States and other developed countries, it is
thought that fewer than 10% of all cancers are linked to infectious
agents. In developing countries, infections can account for as much as
20% of all cancers.
Some infections may cause long-term inflammation, suppress a
person's immune system, or directly affect a cell's DNA. Any of these
pathways may lead to a higher risk of cancer.
Even though the infections described below can raise a
person's risk of certain types of cancer, most people with these
infections never develop cancer.
The risk of developing a cancer is also influenced by other
factors. For example, infection with Helicobacter pylori
(H. pylori)
bacteria may increase your risk of developing stomach cancer, but what
you eat, whether or not you smoke, and other factors also influence
your risk.
Although the infections that influence cancer risk are usually
contagious (especially viral infections), cancer itself is not a
contagious disease. A healthy person cannot get cancer from someone who
has the disease.
Viruses
Viruses are very small organisms -- most cannot even be seen
with an ordinary microscope. They are made up of a small group of genes
(in the form of DNA or RNA) surrounded by a protein coating. Viruses
cannot reproduce on their own. They need to enter a living cell and
"hijack" the cell's machinery to make more viruses. Some viruses do
this by inserting their own DNA (or RNA) into that of the host cell.
When this insertion affects the host cell's genes, it may push the cell
toward becoming cancerous.
Several viruses are now known or suspected of being linked
with cancer in humans. Our growing knowledge of the role of viruses as
a cause of cancer may lead to vaccines that prevent or treat certain
human cancers in the future.
Human papilloma viruses (HPVs)
HPVs are a group of over 100 related viruses that can cause
warts on the skin, mouth, genital organs, and larynx. They are spread
by contact (touch), including through sex. HPV infections are very
common in people who are sexually active. There are no effective
treatments for HPV other than removing or destroying cells that are
known to be infected. Most genital HPV infections go away over time
with the help of the body's immune system.
Certain types of HPV are the main cause of cervical cancer,
which is the second most common cancer among women worldwide. It is
much rarer in the United States because of the use of the Pap test.
This test can detect pre-cancerous changes in cells in the cervix that
might be due to HPV infection, which can then be treated, if needed.
This treatment can prevent the development of cancer.
While nearly all women who develop cervical cancer shows signs
of HPV infection, most women infected with HPV will not develop
cervical cancer. Even though doctors can test women for HPV, there is
no treatment for the HPV infection itself. If the HPV causes abnormal
cells to start growing, these cells can be treated. Women with HPV
infection may be checked for abnormal cells more often than those who
don't have it. For more information, see the American Cancer Society
document, Thinking About Testing for HPV?.
HPVs also have a role in causing some cancers of the penis,
anus, vagina, and vulva. They have also been linked to cancers of the
mouth, throat, head, and neck. Again, while HPVs have been linked to
these cancers, most people infected with HPV never develop cancer.
Smoking and drinking, which are also linked with these cancers, may
work together with HPV to increase cancer risk.
Vaccines against the types of HPV that cause cancer are now
under development. One of these vaccines (Gardasil®)
has been
shown to help protect against infection from 2 of these HPV types, and
is now approved for use in females aged 9 to 26. Because it is still
fairly new, it is not yet known how well it will protect against
cervical cancer. Further studies of this vaccine and others like it are
still under way.
Epstein-Barr virus (EBV)
EBV is another type of herpes virus. It is probably best known
for causing infectious mononucleosis, also known as "mono" or the
"kissing disease." In addition to kissing, it can be passed from person
to person by coughing, sneezing, or sharing drinking or eating
utensils. Most people in the United States are infected with EBV before
the age of 20, although not everyone develops the symptoms of mono. As
with other herpes viruses, the virus remains in the body throughout
life, but after the first few weeks of infection most people never have
any other symptoms.
EBV infects and stays in certain white blood cells in the body
called B lymphocytes (also called B cells). Infection with EBV
increases a person's risk of getting nasopharyngeal cancer (cancer
of
the area in the back of the nose) and certain types of fast-growing
lymphomas such as Burkitt
lymphoma. It may also be linked to Hodgkin
disease and some cases of stomach cancer.
These cancers are more common
in Africa and parts of Southeast Asia. Overall, very few people who
have been infected with EBV will ever develop these cancers.
Hepatitis B virus (HBV) and hepatitis C
virus (HCV)
HBV and HCV cause viral hepatitis, a type of liver infection.
While other viruses can also cause hepatitis (hepatitis A virus, for
example), only HBV and HCV can cause chronic (long-term) infections
that increase a person’s chance of developing liver cancer.
In the United States, about 30% of liver cancers are related to HBV or
HCV infection. This number is much higher in certain other countries,
where both the infections and liver cancer are much more common.
HBV and HCV are spread from person to person in much the same
way as HIV (see section on HIV below) -- through sharing needles,
unprotected sex, or childbirth. They can also be passed on through
blood transfusions, although this has rarely happened in the United
States since blood products began to be tested for these viruses.
Of the 2 viruses, infection with HBV is more likely to cause
symptoms, such as a flu-like illness and a yellowing of the eyes and
skin (jaundice). But most people recover completely from HBV infection
within a few months. Only a very small percentage go on to become
chronic carriers. These people have a higher risk for liver cancer.
HCV, on the other hand, is less likely to cause symptoms. But
most people with HCV develop chronic infections, which are more likely
to lead to liver damage or even cancer. This means that many of the
estimated 3.2 million people in the United States who have chronic HCV
infection do not know they have it.
While there are few drugs effective in treating hepatitis B or
C, a vaccine is available to prevent HBV infection. In the United
States, the vaccine is recommended for all children and for adults who
are at risk, such as healthcare workers and injection drug users.
Unfortunately, there is no vaccine to prevent HCV at this time.
For more information, see the American Cancer Society
Document, Liver Cancer.
Human immunodeficiency virus (HIV)
HIV, the virus that causes acquired immune deficiency syndrome
(AIDS), does not appear to cause cancers directly. But HIV infection
increases a person's risk of getting several types of cancer,
especially some linked to other viruses such as HHV-8 (see section
below) and HPV.
HIV is acquired through intimate contact with blood, vaginal
secretions, breast milk, or semen from an HIV-infected person. Known
routes of spread include:
- unprotected sex (oral, vaginal, or anal) with an
HIV-infected person
- injections with needles or injection equipment
previously used on an HIV-infected person
- prenatal and perinatal (during birth) exposure of
infants from mothers with HIV
- breast-feeding by mothers with HIV
- transfusion of blood products containing HIV (blood
has been tested since 1985)
- organ transplants from an HIV-infected person
(donors are now tested for HIV)
HIV is not spread by insects, through water, or by casual
contact such as talking, shaking hands, hugging, sneezing, sharing
dishes, sharing a bathroom or kitchen, sharing telephones, or sharing
computers.
HIV infects and destroys white blood cells known as helper T
cells, which weakens the body's immune system. When the body is less
able to fight off infections, other viruses such as HPV may be able to
cause more damage to the cells. This damage may trigger cancer.
Many scientists believe that the immune system is also
important in attacking and destroying newly formed cancers. So a weak
immune system may allow new cancers to survive long enough to become a
serious, life-threatening tumor.
HIV infection has been linked to a higher risk of developing
of Kaposi sarcoma, invasive cervical cancer, and certain kinds of
lymphoma,
especially non-Hodgkin lymphoma and central nervous system
lymphoma. Anti-HIV drugs may be used to reduce the risk of Kaposi
sarcoma and cervical cancer.
Other forms of cancer that may be more likely to develop in
people with HIV infection include:
- invasive anal cancer
- Hodgkin disease
- lung cancer
- cancer of the mouth and throat
- cancer of the testicles
- skin cancers, including basal cell, squamous cell,
and even malignant melanomas
For more information, see the American Cancer Society
document, HIV Infection and AIDS.
Human herpes virus 8 (HHV-8)
HHV-8, also known as Kaposi sarcoma-associated herpes virus
(KSHV), has been found in nearly all tumors in patients with Kaposi
sarcoma (KS). KS is a rare, slow-growing cancer that often appears as
reddish-purple or blue-brown tumors just underneath the skin. KS has
been known to exist in central Africa and the Middle East for some
time, but was rare in the United States until it started appearing in
patients with AIDS in the early 1980s. The number of people with KS has
dropped in the United States since peaking in the early 1990s, most
likely due to better treatment of HIV infection.
HHV-8 is transmitted sexually and appears to be spread by
other means as well. Blood tests show less than 10% of the US
population is infected with this virus. HHV-8 does not appear to cause
disease in most healthy people. In the United States, almost all people
who develop KS have other conditions that have affected their immune
system, such as infection with the HIV or immune suppression after an
organ transplant.
HHV-8 is related to other herpes viruses, such as the viruses
that cause cold sores and genital herpes, Epstein-Barr virus (EBV), and
cytomegalovirus (CMV). But these other viruses are not the same as
HHV-8 and do not cause KS. Like other herpes virus infections, HHV-8
infections never go away, even when there are no signs of disease.
Researchers are not yet sure how HHV-8 contributes to the
development of KS. Because many more people are infected with HHV-8
than ever develop the disease, it is likely that other factors are also
needed for KS to develop. As mentioned above, having a weakened immune
system appears to be one such factor. For more information, see the
American Cancer Society document, Kaposi Sarcoma.
HHV-8 infection has been linked to some rare blood cancers,
such as primary
effusion lymphoma. The virus has also been found in
many people with multicentric
Castleman disease, an overgrowth of lymph
nodes that acts very much like lymphoma. (For more information, see the
American Cancer Society document, Castleman Disease.)
Further study is
needed to better understand the role of HHV-8 in these diseases.
Human T-lymphotrophic virus-1 (HTLV-1)
HTLV-1 has been linked with a type of lymphocytic leukemia and
non-Hodgkin lymphoma called adult
T-cell leukemia/lymphoma (ATL). This
cancer is found mostly in southern Japan, the Caribbean, Central
Africa, parts of South America, and in some immigrant groups in the
southeastern United States. In addition to ATL, the virus also causes a
form of degenerative nerve disease called tropical spastic paraparesis
(TSP), which is most common in Japan and in the Caribbean basin.
HTLV-1 belongs to a class of viruses called retroviruses.
These viruses use RNA (instead of DNA) for their genetic code. To
reproduce, they must go through an extra step using an enzyme called
reverse transcriptase. This allows them to change their RNA genes into
DNA. Some of the new DNA genes can then become part of the chromosomes
of the human cell infected by the virus. This can change the genes
(cause genetic mutations) in human cells that normally control how
often the cell divides. This change can sometimes cause cancer.
Retroviruses have long been known to cause leukemia in some animals.
HTLV-1 is something like HIV, since it is another human
retrovirus. But HTLV-1 cannot cause AIDS. In humans, HTLV-1 can be
spread in the same ways as HIV:
- unprotected sex with an HTLV-1-infected partner
- injection with a needle or injection equipment
after an infected person has used it
- blood transfusion from an infected donor (blood
donations are now tested for this virus in developed countries)
- from infected mother to child during pregnancy,
childbirth, or breastfeeding
Not everyone exposed to the virus becomes infected. For
example, mothers infected with HTLV-1 have about a 10% to 30% chance of
passing on the virus to their children.
A survey of people coming to donate blood in several locations
around the United States showed that, overall, about 1 out of every
4,000 people had HTLV-1 (about 0.025%). Around 2% to 10% of people who
use intravenous drugs or who have gotten multiple transfusions become
infected with HTLV-1. Screening of all donated blood in the United
States has greatly reduced the chance of infection through transfusion
and has helped control the potential spread of HTLV-1 infection.
Once infected, a person's chance of developing adult T-cell
lymphoma can be up to about 5%, usually after a long period with no
symptoms (20 or more years).
Viruses with uncertain or unproven links to
cancer in humans
Feline leukemia virus (FeLV)
FeLV causes feline leukemia, a fatal disease in American
domestic cats. A vaccine to protect cats against FeLV is available, and
all healthy cats can be immunized against feline leukemia. FeLV has not
been known to be transmitted to humans.
Simian virus 40 (SV40)
SV40 is a virus that usually infects monkeys. Some polio
vaccines prepared between 1955 and 1963, which were produced from
monkey cells, were found to be contaminated with SV40.
Some recent studies have raised the possibility that infection
with SV40 might increase a person's risk of developing mesothelioma (a
rare cancer of the lining of the lungs or abdomen), as well as some
brain cancers, bone cancers, and lymphomas.
Scientists have found that intentional infection of some lab
animals, such as hamsters, with SV40 causes mesotheliomas to develop.
Researchers have also noticed that SV40 can cause mouse cells grown in
the lab to become cancerous, and asbestos increases the cancer-causing
effect of SV40 on these cells. Other researchers have studied biopsy
specimens of certain human cancers and found fragments that appear to
be SV40 DNA. But not all researchers have found this, and fragments
much like these can also be found in non-cancerous human tissues.
So far, the largest studies looking at this issue in humans
have not found any increased risk for mesothelioma or other cancers
among people who got the contaminated vaccines as children. For
example, the recent increase in lung mesothelioma cases has been seen
mainly in men aged 75 and older, most of whom would not have received
the vaccine. Another observation is that, among the age groups who were
known to have gotten the vaccine, mesothelioma rates have actually gone
down. And even though women were just as likely to have had the
vaccine, many more men continue to be diagnosed with mesothelioma. But
the peak age range for diagnosis of mesothelioma is 50 to 70 years.
Some researchers have pointed out that this issue may remain unresolved
until more of the people exposed to SV40 between 1955 and 1963 get into
that age range.
Recent findings have suggested that the number of people
exposed to SV40 may have been underestimated in earlier studies. Some
of the oral polio vaccine that was made in Eastern Europe and used
throughout the world may have contained SV40 up until the late 1970s.
Research into this important topic is still under way.
Bacteria
Helicobacter pylori (H. pylori)
While stomach
cancer is fairly rare in the United States, it
is the fourth most common cancer worldwide. Long-term infection of the
stomach with H. pylori
may lead to chronic inflammation and damage to
the inner layer of the stomach, including ulcers. Some of these changes
could lead to cancer over time, especially cancer in the lower part of
the stomach. H. pylori
infection is also linked with some types of
lymphoma of
the stomach.
More than half of all cases of stomach cancer are thought to
be linked to H. pylori
infection. But most people who have these
bacteria in their stomachs never develop cancer.
About 1 in 3 adults has evidence of infection with H. pylori,
and the rate of infection is higher in older age groups. Researchers
aren't exactly sure how H.
pylori may be spread from one person to
another, but a likely route of spread is through a fecal-oral route,
such as through contaminated water sources. In fact, contaminated well
water has been linked to H.
pylori infection in the United States.
Because the bacteria's DNA is found in saliva, it may also be
transmitted from mouth to mouth.
Other factors also play a role in whether or not someone
develops stomach cancer. For example, nitrites are substances commonly
found in cured meats, some drinking water, and certain vegetables. They
can be converted by certain bacteria, such as H. pylori, into
compounds
that have been found to cause stomach cancer in animals.
Antibiotics and other medicines can be used to treat H. pylori
infections. It is not yet known if people with chronic H. pylori
infection of their stomach lining but no symptoms should be treated for
this infection. Some doctors believe that patients with H. pylori who
are at high risk of stomach cancer should be treated whether or not
they have symptoms. These issues are still being studied.
Doctors have given antibiotics to patients who have had
superficial stomach cancers removed in order to get rid of H. pylori
infection. This seems to have prevented new stomach cancers in those
patients.
Chlamydia trachomatis
Chlamydia
trachomatis is a relatively common kind of bacteria
that can infect the female reproductive system. It is spread by sexual
contact. Although infection may cause symptoms, more than 2 out of 3
women have no symptoms. This means that most women with chlamydia do
not know they are infected unless samples taken when they have a Pap
test are also studied for this type of bacteria. It is very common in
younger women who are sexually active, and may persist for years unless
it is detected and treated.
Some studies suggest that women whose blood test results show
past or current chlamydia infection are at greater risk for cervical
cancer than are women with a negative blood test. Studies
have not
shown that chlamydia by itself can cause cancer. But it may work with
HPV in some way that promotes cancer growth. One possible explanation
suggested by 2 recent studies is that chlamydia may affect how long
cancer-promoting HPV stays in the cervix. Researchers found that women
who had chlamydia along with HPV were more likely to still have HPV
when they were re-tested later than the women who had not had
chlamydia. Although further studies are needed to confirm these
findings, there is already good reason to avoid this infection and to
have it treated with antibiotics when it is found. Long-term chlamydia
infection is known to be a cause of pelvic inflammation that can lead
to infertility. Like other sexually transmitted diseases that irritate
or ulcerate the genital area, chlamydia can also increase the risk of
becoming infected with HIV during exposure to an HIV-infected sexual
partner.
Other bacteria
Chlamydia
psittaci is being studied as a possible cause of
cancer. It is best known for causing a mild infection sometimes known
as "parrot fever" when the germ is inhaled from dried bird droppings or
feather dust. (This germ is not like the type of chlamydia discussed
above and is not sexually transmitted.) Two recent studies have
suggested that Chlamydia
psittaci bacteria may also be involved in a
rare cancer of the eyes known as ocular
adnexal mucosa-associated
lymphoid tissue (MALT) lymphoma. In some cases, this
lymphoma started
to improve when the patient was given an antibiotic to treat the
bacteria, but this did not happen in others. Further research is needed
to learn more about the link between the bacteria and the cancer.
Other studies have suggested that infection with Borrelia
burgdorferi (the bacterium that causes Lyme disease) may
be linked with
MALT lymphoma of the skin. And Campylobacter
jejuni may be linked with
MALT lymphomas of the digestive tract. Further research is needed to
clarify these links.
Chlamydia
pneumoniae is a type of bacteria that can cause lung
infections (pneumonia). In several studies, more people with lung
cancer had evidence of previous infection with this germ than those who
did not have lung cancer. Even though it looks like there may be a
link, these types of studies do not show that the bacteria caused the
cancer. Studies are under way to explore the chance that this germ can
increase the risk of lung cancer.
Parasites
Certain parasitic worms that can live inside the human body
can also raise the risk of developing some kinds of cancer. While these
organisms are not found in the United States, they can be a concern for
people who live in or travel to other parts of the world.
Opisthorchis
viverrini and Clonorchis
sinensisare liver flukes
(a type of flatworm) that have been linked to increased risk of
developing cancer of
the bile ducts (the tubes that connect the liver
to the intestines). These infections come from eating raw or
undercooked fish. They are found mostly in East Asia and are rare in
other parts of the world.
Schistosoma
haematobium is a parasite found in the water of
developing countries of Africa and Asia. Infection with this parasite
(an illness called schistosomiasis) has been linked to bladder cancer.
Possible links to other types of cancer are now being studied as well.
Additional resources
More information from your American Cancer
Society
We have selected some related information that may also be
helpful to you. These materials may be ordered from our toll-free
number, 1-800-ACS-2345.
National organizations and Web sites*
In addition to the American Cancer Society, other sources of
patient information include:
Centers for Disease Control and Prevention (CDC)
Toll-free number: 1-800-CDC-INFO (1-800-232-4636)
Web site: www.cdc.gov
National Cancer Institute
Toll-free number: 1-800-4-CANCER (1-800-422-6237)
Web site: www.cancer.gov
*Inclusion on
this list does not imply endorsement by the
American Cancer Society.
No matter who you are, we can help. Contact us anytime, day or
night, for information and support. Call us at 1-800-ACS-2345 or
visit
www.cancer.org.
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Last medical review: 05/19/2008
Last revised: 10/20/2008
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