Why does poliovirus cause muscle weakness or flaccid paralysis?

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Polio vaccine — polio A health care worker giving a polio vaccine to a child in Katsina state, Nigeria, 2014.

Poliovirus causes muscle weakness and flaccid paralysis by attacking the central nervous system, specifically by inflaming and destroying motor neurons located in the spinal cord and brainstem.1

Motor neurons are essential for transmitting signals from the central nervous system to the muscles to trigger contraction. When the virus destroys these cells, the transmission of these vital motor impulses is blocked. Because nerve cells cannot regenerate, the destruction of these neurons prevents the muscles from receiving the necessary signals to function, resulting in them becoming limp and unable to contract.1

The severity of the paralysis depends on the location of the viral attack and the total number of nerve cells destroyed. While cells that are only mildly injured may recover their function, those that are completely destroyed lead to permanent paralysis and the subsequent atrophy of the affected muscles.1