Mass psychogenic illness is a phenomenon that may rarely occur as a consequence of a vaccination campaign. Such an event recently occurred in Pakistan during a polio mass vaccination campaign and involved around 30,000 children. Social media played an important part in spreading the information about the phenomenon. Once started, it is difficult to stop such an outbreak, and it may seriously damage public trust in government. It has the potential for disrupting the entire national vaccination programme. Trust in vaccination can be hard to gain but is easily lost.