Final answer:
The most severe polio symptoms arise when the virus attacks nerves, leading to damage and paralysis of the muscles they control. Poliovirus replication in the CNS disrupts motor neurons in the spinal cord and brain stem, which can result in severe respiratory dysfunction in the worst cases.
Step-by-step explanation:
The most severe symptoms from polio are caused by the pathogen attacking nerves, damaging them and the muscles they are attached to. This is because poliovirus specifically targets the motor neurons.
Poliovirus is a highly contagious virus that often starts as an intestinal infection but becomes more serious if it progresses to the nervous system. Once the virus enters the mouth, it initially replicates in the pharynx and gastrointestinal tract, before potentially spreading to the central nervous system. If it reaches the central nervous system (CNS), the virus attacks motor neurons in the spinal cord, brain stem, or motor cortex, leading to cell destruction and, consequently, to flaccid paralysis. In some severe cases, the respiratory muscles are affected, necessitating mechanical ventilation, such as with an 'iron lung', to assist with breathing.
The poliovirus can cause long-lasting damage because motor neurons do not readily regenerate, and thus paralysis can persist. However, it is critical to note that fewer than 5% of individuals infected with poliovirus will show noticeable symptoms, and only about 1 in 200 infections leads to severe neurological disease, with the majority of infections being asymptomatic or resulting in mild symptoms such as nausea, fever, and headache.