Polio

Poliomyelitis, often called polio, is a highly infectious disease caused by the poliovirus.

This virus belongs to a group called enteroviruses, which are known to colonize the human gastrointestinal tract. While the disease can be quite severe, it is important to note that it occurs naturally only in humans. The term itself comes from Ancient Greek, meaning "grey matter inflammation," referring to the part of the spinal cord the virus often affects.

Understanding how this virus moves through the body and how we fight it is a major part of modern medicine.

The virus enters the body through the mouth, typically via the fecal–oral route. This happens when a person ingests food or water contaminated by human feces, often due to poor hygiene. It can also spread through the oral–oral route. Once inside, the virus targets the oropharynx and the intestinal mucosa. It enters cells by binding to a specific receptor called CD155. After entering, the virus hijacks the host cell's machinery to replicate itself.

This replication occurs within the gastrointestinal cells for about one week before spreading to the lymph nodes and the bloodstream. This stage, where the virus is in the blood, is known as viremia.

Most people who catch the virus do not realize they are sick. Approximately 75% of cases are asymptomatic, meaning they show no symptoms at all. About 25% of people experience a minor illness, sometimes called abortive poliomyelitis. These symptoms include a sore throat, fever, and general discomfort, but usually pass within one or two weeks. However, in about 1% of cases, the virus migrates from the gut into the central nervous system (CNS). This can cause nonparalytic aseptic meningitis, which involves inflammation of the membranes surrounding the brain and spinal cord. Patients might feel headaches, neck stiffness, or abdominal pain during this time.

In a small number of cases, the infection progresses to paralytic poliomyelitis. This occurs when the virus destroys motor neurons, the nerve cells that control muscle movement. This leads to acute flaccid paralysis, where muscles become weak, floppy, and lose control.

Depending on where the damage occurs in the CNS, doctors classify the paralysis into three types. Spinal polio affects the spinal cord, while bulbar polio affects the brain stem. Bulbospinal polio involves both regions.

If the paralysis affects the muscles used for breathing, it can be fatal. Between 2% and 10% of those with paralytic polio may die from this cause.

Humans have lived with polio for thousands of years, with ancient art even depicting the disease.

The English physician Michael Underwood first recognized it as a distinct medical condition in 1789. Later, the Austrian immunologist Karl Landsteiner identified the actual poliovirus in 1909. By the late 19th century, major outbreaks began appearing in Europe and the United States. During the 20th century, polio became one of the most feared childhood diseases globally. This changed rapidly after the introduction of vaccines in the 1950s.

Vaccines are the most effective way to prevent the disease. There are two main types used to create immunity. The injected polio vaccine (IPV) uses inactivated, or killed, poliovirus. The oral polio vaccine (OPV) uses attenuated, or weakened, live virus.

While OPV is effective, in rare instances, the weakened virus can mutate back into a virulent form in areas with low vaccine coverage. This is known as circulating vaccine-derived poliovirus (cVDPV). To address this, a more genetically stable version called nOPV2 was granted full licensure by the World Health Organization in December 2023.

The impact of vaccination has been massive. In 1988, there were an estimated 350,000 cases of wild poliovirus worldwide. By 2022, that number had dropped to only 30 confirmed cases.

Today, wild poliovirus remains endemic in only two countries: Pakistan and Afghanistan. This progress shows how global health efforts can nearly eliminate a dangerous pathogen. Even as we fight the virus, some survivors deal with post-polio syndrome, which brings back muscle weakness many years after the initial recovery.