Poliomyelitis

Poliomyelitis, or polio, is an infection caused by the poliovirus, a small, non-enveloped, acid-resistant virus whose genetic material is a single strand. It spreads by the fecal-oral route and also the oral-oral route, that is, through contact with stool or with secretions from the mouth.

Its path through the body has stages. It first multiplies in the oropharynx, the part of the throat behind the mouth, and in the digestive tract. In most cases the story ends there. In a minority, the virus passes into the blood and from there enters the central nervous system, either crossing the blood-brain barrier, the filter that protects the brain, or traveling backward inside the nerves.

What it does once it arrives explains the paralysis. It destroys the alpha motor neurons of the anterior horn of the spinal cord, the neurons responsible for telling the muscle to contract, and also cranial nerve nuclei in the brainstem. It is a cut wire at the panel: the muscle is still intact, but nothing is talking to it anymore. Over time, the muscles left without innervation waste away, and that includes those of the limbs, the trunk and the breathing muscles, among them the diaphragm.