=Herd Immunity= [image:http://i.imgur.com/988TY4x.png?1] Herd immunity is a form of immunity that occurs when a large proportion of a population have been vaccinated. Vaccinated individuals who are immune act as a barrier to the transmission of the disease. If enough individuals are vaccinated it becomes very difficult for the disease to spread throughout the population. Unvaccinated individuals are '''not''' immune, but are offered some protection by vaccinated individuals who reduce the likelihood that they will come in contact with the disease. Herd immunity is particularly crucial for protecting people who cannot be vaccinated. These include children who are too young to be vaccinated, people with immune system problems, and those who are too ill to receive vaccines (such as some cancer patients). The proportion of the population which must be immunised in order to achieve herd immunity varies for each disease but the underlying idea is simple: once enough people are protected, they help to protect vulnerable members of their communities by reducing the spread of the disease. However, when immunisation rates fall, herd immunity can break down leading to an increase in the number of new cases. For example, measles outbreaks in the UK and pertussis outbreaks in the US have been attributed to declining herd immunity.