Question: How many viral infections can vaccines prevent?
About the article "Virus", section "Vaccines".

Candidate passages:
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.
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. 
Viral epidemiology is the branch of medical science that deals with the transmission and control of virus infections in humans.
Transmission of viruses can be vertical, which means from mother to child, or horizontal, which means from person to person.
Examples of vertical transmission include hepatitis B virus and HIV, where the baby is born already infected with the virus.
Another, more rare, example is the varicella zoster virus, which, although causing relatively mild infections in humans, can be fatal to the foetus and newborn baby.
Horizontal transmission is the most common mechanism of spread of viruses in populations.
Transmission can occur when: body fluids are exchanged during sexual activity, e.g., HIV; blood is exchanged by contaminated transfusion or needle sharing, e.g., hepatitis C; exchange of saliva by mouth, e.g., Epstein-Barr virus; contaminated food or water is ingested, e.g., norovirus; aerosols containing virions are inhaled, e.g., influenza virus; and insect vectors such as mosquitoes penetrate the skin of a host, e.g., dengue.
The rate or speed of transmission of viral infections depends on factors that include population density, the number of susceptible individuals, (i.e., those not immune), the quality of healthcare and the weather.
Epidemiology is used to break the chain of infection in populations during outbreaks of viral diseases.
Control measures are used that are based on knowledge of how the virus is transmitted.
It is important to find the source, or sources, of the outbreak and to identify the virus.
Once the virus has been identified, the chain of transmission can sometimes be broken by vaccines.
When vaccines are not available, sanitation and disinfection can be effective.
Often, infected people are isolated from the rest of the community, and those that have been exposed to the virus are placed in quarantine.
To control the outbreak of foot-and-mouth disease in cattle in Britain in 2001, thousands of cattle were slaughtered.
Most viral infections of humans and other animals have incubation periods during which the infection causes no signs or symptoms.
Incubation periods for viral diseases range from a few days to weeks, but are known for most infections.
Somewhat overlapping, but mainly following the incubation period, there is a period of communicability — a time when an infected individual or animal is contagious and can infect another person or animal.
This, too, is known for many viral infections, and knowledge of the length of both periods is important in the control of outbreaks.
When outbreaks cause an unusually high proportion of cases in a population, community, or region, they are called epidemics.
If outbreaks spread worldwide, they are called pandemics. 
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. 
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.