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As individuals, vaccination is an important way of preventing development of disease in our bodies and stopping us from being ill from infections. On a wider scale, if a group of people are vaccinated against a disease it makes it more difficult for the microbe causing the disease to find an unprotected person to infect. If enough people are immunised then those who aren't can still be protected against the disease. This is know as herd immunity. For example, with a highly infectious disease such as measles, 95% of the population need to be immunised against the virus for this effect to work (see the Health Promotion England's Immunisation Website for more details. Vaccination programs can eventually kill off some diseases. A World Health Organisation (WHO) vaccination program successfully eradicated smallpox from the world and it is hoped that the same will be possible with polio. In fact, the WHO has recently declared Europe as polio-free (See the WHO Press release for more details). However, not all diseases can be eradicated by vaccination as some can be carried by people without any ill effects, e.g. diptheria.
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