In many respects, the neurological phase of infection is thought to be an accidental diversion of the normal gastrointestinal infection. The mechanisms by which poliovirus enters the CNS are poorly understood. Three nonmutually exclusive hypotheses have been suggested to explain its entry. All theories require primary viremia. The first hypothesis predicts that virions pass directly from the blood into the central nervous system by crossing the blood–brain barrier independent of CD155. A second hypothesis suggests that the virions are transported from peripheral tissues that have been bathed in the viremic blood, for example muscle tissue, to the spinal cord through nerve pathways via retrograde axonal transport. A third hypothesis is that the virus is imported into the CNS via infected monocytes or macrophages.