Wild poliovirus (WPV) comprises 3 serotypes (1, 2, 3), which may infect and destroy spinal the lower motor neurons.
PV is shed via salivary droplets and feces, and it is transmitted person-to-person.
One case of polio occurs following ~200 (WPV type 1) to ~1000 (type 2 or 3) WPV infections, making one case the “tip of the iceberg” of widespread infection.
Infection induces long-lasting type-specific immunity, protecting from disease on re-infection, but not from re-infection itself.
In low-income countries, polio occurs early in life and immunity plateaus at 5 years of age, with almost 100%; in richer countries, the age of polio has shifted to older ages since the 1940s.
Most WPV-infected persons remain asymptomatic; a small proportion have short fever with upper respiratory or gastrointestinal symptoms.
A few may develop an acute onset of paralysis of skeletal muscles due to loss of lower motor neurons (poliomyelitis) with a case-fatality rate of 5%–20%; bulbar involvement increases the risk of death; cortical functions (other than emotional) remain unaffected.
Recurrence of pain and worsening of residual motor power may occur 3–4 decades later (post-polio syndrome).
