Question: What is the number of viral infections that vaccines can prevent?
About the article "Virus", section "Vaccines".

Candidate passages:
Prevention and treatment of viral disease in humans and other animals.
Vaccination is a way of preventing diseases caused by viruses.
Vaccines simulate a natural infection and its associated immune response, but do not cause the disease.
Their use has resulted in a dramatic decline in illness and death caused by infections such as polio, measles, mumps and rubella.
Vaccines are available to prevent over thirteen viral infections of humans and more are used to prevent viral infections of animals.
Vaccines may consist of either live or killed viruses.
Live vaccines contain weakened forms of the virus, but these vaccines can be dangerous when given to people with weak immunity.
In these people, the weakened virus can cause the original disease.
Biotechnology and genetic engineering techniques are used to produce "designer" vaccines that only have the capsid proteins of the virus.
Hepatitis B vaccine is an example of this type of vaccine.
These vaccines are safer because they can never cause the disease. 
Vaccination is a cheap and effective way of preventing infections by viruses.
Vaccines were used to prevent viral infections long before the discovery of the actual viruses.
Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps and rubella.
Smallpox infections have been eradicated.
Vaccines are available to prevent over thirteen viral infections of humans, and more are used to prevent viral infections of animals.
Vaccines can consist of live-attenuated or killed viruses, or viral proteins (antigens).
Live vaccines contain weakened forms of the virus, which do not cause the disease but, nonetheless, confer immunity.
Such viruses are called attenuated.
Live vaccines can be dangerous when given to people with a weak immunity, (who are described as immunocompromised), because in these people, the weakened virus can cause the original disease.
Biotechnology and genetic engineering techniques are used to produce subunit vaccines.
These vaccines use only the capsid proteins of the virus.
Hepatitis B vaccine is an example of this type of vaccine.
Subunit vaccines are safe for immunocompromised patients because they cannot cause the disease.
The yellow fever virus vaccine, a live-attenuated strain called 17D, is probably the safest and most effective vaccine ever generated.
A cancer vaccine is a vaccine that treats existing cancer or prevents the development of cancer in certain high-risk individuals.
Vaccines that treat existing cancer are known as "therapeutic" cancer vaccines.
There are currently no vaccines able to prevent cancer in general.
Some types of cancer, such as cervical cancer and some liver cancers, are caused by viruses (known as oncoviruses), and traditional vaccines against those viruses, such as HPV vaccine and hepatitis B vaccine, will prevent those types of cancer.
These anti-viral vaccines are not further discussed in the rest of this article.
Other cancers are to some extent caused by bacterial infections (e.g.
stomach cancer and "Helicobacter pylori") and traditional vaccines against cancer-causing bacteria are also not discussed in this article.
Scientists have also been trying to develop vaccines against existing cancers.
Some researchers believe that cancer cells routinely arise and are destroyed by the healthy immune system; cancer forms when the immune system fails to destroy them. 
Immunoprevention of tumors caused by viruses or other infectious agents aims at preventing or curing infection before the onset of cancer.
Effective vaccines are available for use in humans.
Some tumor types in humans and in animals are the consequence of viral infections.
In humans the most frequent viral tumors are liver cancer (also called hepatocellular carcinoma), arising in a small proportion of patients with chronic infection by hepatitis B virus (HBV) or hepatitis C virus (HCV), and carcinoma of the uterine cervix (also called cervical cancer), caused by human papilloma virus (HPV).
Altogether these two tumors make 10% of all human cancers, affecting almost one million new patients each year worldwide.
The HBV vaccine, now in worldwide use, was shown to reduce the incidence of liver carcinoma.
Cancer immunoprevention by the HBV vaccine can be thought of as a beneficial side effect of vaccine developed and used to prevent hepatitis B.
This is not the case with HPV vaccines, which were primarily developed for cancer prevention.
Clinical trials showed that HPV vaccines can prevent HPV infection and carcinogenesis almost completely; these results led to vaccine approval by regulatory agencies in USA and Europe. 
Both helper and cytotoxic T-cells can control viral infections by secreting interferons.
Cytotoxic T cells usually kill virally infected cells.
However, they can also be stimulated to secrete antiviral cytokines such as IFN-γ and TNF-α, which don’t kill the cell but place severe limitations on viral infection by down-regulating the expression of viral components.
DNA vaccinations can thus be used to curb viral infections by non-destructive IFN-mediated control.
This has been demonstrated for the hepatitis B virus.
IFN-γ is also critically important in controlling malaria infections, and should be taken into consideration when developing anti-malarial DNA vaccines.