 Generally speaking, conspiracy theories are theories according to which there exists some type of secret collusion for any type of purpose. In practice, the term mostly refers to implausible such theories, used to explain social facts that have other, considerably more plausible explanations. Many pseudosciences are connected with conspiracy theories. For instance, one of the difficulties facing anti-vaccinationists is that they have to explain the overwhelming consensus among medical experts that vaccines are efficient. This is often done by claims of a conspiracy: 