Vaccines contain dead or weakened versions of the disease they are trying to prevent – a vaccine-virus. After immunisation, the vaccine-virus will temporarily live within the vaccinated. During this time, the vaccine-virus will be excreted and in areas of low sanitisation, it will spread to other people and offer passive immunisation before dying out. In the worst case scenarios, it will infect an immunocompromised person and result in the disease spreading again. If all children in a community are immunised, the vaccine-virus has no one to infect and dies away.