To explain the data above, two hypotheses were proposed. The first is known as the Poliomyelitis hypothesis and suggests that infection at a young age confers immunity but adult infection increases MS risk. The second is known as the Prevalence hypothesis, and suggests that MS is caused by a pathogen that is more common in regions with high rates of MS. This pathogen would be widespread and cause an asymptomatic (latent) infection in most individuals. Only rarely and years after the primary infection does this hypothetical agent cause the neurological symptoms of MS. A third hypothesis essentially combines these two and also suggests the involvement of multiple pathogens. The third may best apply to the epidemiological data.