Infection: Polio

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Infection: Polio

Causes and Symptoms
The Future
For more information


Polio is an infectious disease caused by a virus that normally lives in the human digestive tract. About 90 percent of persons infected by the virus have no symptoms at all; in the other 10 percent, the polio virus causes symptoms ranging from a mild flu-like illness to paralysis of the lower limbs or death from paralysis of the muscles that control breathing.


Polio was a greatly feared disease in the early part of the twentieth century because of epidemics that used to occur in the summer and fall of every year. Ironically, these epidemics were a byproduct of the improved sanitation and standards of cleanliness in large cities in Europe and the United States toward the end of the nineteenth century. Although polio had been known since ancient Egypt, the disease was relatively uncommon. The reason for this was that most young children before the nineteenth century were exposed to the polio virus early in life and thereby acquired lifetime immunity. Better hygiene, however, meant that infants and young children had fewer opportunities to encounter and develop immunity to polio. Exposure to poliovirus was therefore delayed until later in childhood or adult life, when it was more likely to cause paralysis or death.

The first recorded outbreak of polio in the United States occurred in Louisiana in 1841. Outbreaks were few until the early 1900s, however, when large cities like New York and Boston began to experience annual polio epidemics. In 1916, 27,000 people across the United States fell ill with polio and 6,000 died. The epidemics of the 1940s and 1950s were even worse; the 1952 epidemic was the worst in the country'shistory. There were 58,000 cases reported; 3,145 people died, and 21,000 were left paralyzed. This history helps to explain why the introduction of the Salk vaccine in 1955 was regarded as a miracle. By 1957, the success of the vaccine was reflected in the fact that the annual number of cases fell to 5,600. After the development of the Sabin oral vaccine in the early 1960s, the annual number of polio cases in the United States fell to 160. The last cases of endemic polio in the United States were reported in 1979.

There are four major categories of polio:

  • Asymptomatic: Between 90 and 95 percent of people infected by the polio virus have no symptoms at all.
  • Minor flu-like illness: Between 4 and 8 percent of patients. This minor illness is sometimes called abortive polio.
  • Nonparalytic meningitis: This is a condition that affects about 2 percent of patients. It is characterized by headache; pain in the neck, back and abdomen; fever; vomiting; and irritability.
  • Paralytic polio: Between one in 200 and one in 1,000 patients develop a paralytic polio.


Polio was widespread in the developed countries in the first part of the twentieth century. The epidemics not only became more severe, but also affected adolescents and adults rather than mostly children. The older average age of patients was also marked by increased severity of symptoms. Since the introduction of effective vaccines, paralytic polio is almost unknown in the United States except among recent immigrants and other groups (such as the Amish) that do not routinely participate in community-wide vaccination programs.

Worldwide, polio epidemics are most common in tropical countries during the months of July through September. Both sexes and all races are equally likely to get the disease if they are not protected by immunization.

Some people are more likely than others to develop the paralytic form of the disease if they do become infected:

  • Young children
  • Elderly adults
  • People who engage in hard physical labor or strenuous exercise
  • People who have recently had a tonsillectomy or dental surgery
  • Pregnant women
  • People who travel frequently to areas where polio is still endemic
  • Those with an immune system weakened by HIV or certain types of cancer treatment

Causes and Symptoms

Polio is caused by a virus that enters the mouth through food or water that has been contaminated by fecal matter. It is a contagious illness; anyone living with a recently infected person can become infected. Although people carrying the poliovirus are most contagious for seven to ten days before and after symptoms (if any) appear, they can spread the virus for weeks in their bowel movements.

Once inside the body, the polio virus takes between six and twenty days to incubate. It finds its way to the tissues lining the throat and the intestinal tract, where it multiplies rapidly. After about a week in the intestines, the virus travels to the tonsils and the lymph nodes, where it multiplies further and then enters the bloodstream. In a minority of cases, the virus enters the central nervous system (CNS) from the blood and lymph. It then multiplies in and destroys the nerve cells in the brain known as motor neurons that control the movements of the muscles. The location and severity of the paralytic polio that results when the motor neurons are damaged varies with the part of the CNS that is affected.

Defeating Polio

Many people think of Franklin Delano Roosevelt (1882–1945) as the individual whose struggle called public attention to the need to find a vaccine against polio. Roosevelt acquired polio in August 1921 while vacationing with his family in New Brunswick, Canada. After falling from his boat into the cold waters of the bay on August 9, Roosevelt went swimming and jogging the next day with his children. He felt unwell that night; two days later, his legs were completely paralyzed. In spite of a variety of treatments, Roosevelt remained permanently paralyzed from the waist down until his death in 1945.

Roosevelt refused to allow his condition to prevent him from serving as governor of New York and later as president of the United States. He was instrumental in founding the National Foundation for Infantile Paralysis, later the March of Dimes. Roosevelt's foundation supported the work of Dr. Jonas Salk (1914–1995), the researcher who developed the first safe and effective polio vaccine. Salk's vaccine was licensed in 1955 and was followed by mass vaccination campaigns across

the United States. The Salk vaccine, reformulated for additional potency in 1987, is currently the standard polio vaccine used in the United States.

Between 4 and 8 percent of polio infections are characterized by influenza-like symptoms known as abortive poliomyelitis. People with

this form experience sore throat and fever, nausea, vomiting, abdominal pain, constipation, or diarrhea. Abortive polio is difficult to distinguish from the flu or other viral infections. Patients recover completely in about a week.

Patients with nonparalytic meningitis may experience a brief period of general illness followed by stiffness in the neck, back, or legs. They may also experience other abnormal sensations for a period of two to ten days. As with abortive polio, patients with nonparalytic meningitis recover completely.

Paralytic polio is usually divided into three types, depending on whether the paralysis affects the arms and legs (spinal polio; accounts for 79 percent of cases of paralytic polio); breathing, speaking, and swallowing (bulbar polio; 2 percent of cases); or the limbs as well as breathing and other functions (bulbospinal polio; 19 percent of cases). Bulbar polio is particularly likely to lead to death if the patient is not placed on a respirator because the virus affects the brain stem—the part of the brain that controls heartbeat as well as breathing and other vital functions.


The diagnosis of polio is based on a combination of the patient's history and the type and location of symptoms—particularly such symptoms as a stiff neck, difficulty breathing, or abnormal reflexes. To confirm the diagnosis, samples of the patient's stool, spinal fluid, or throat mucus may be collected and sent to a laboratory for analysis to see whether the sample contains the virus itself. A blood sample early in the infection may also be analyzed for evidence of antibodies to the poliovirus.


There is no cure for polio. Patients with abortive polio or nonparalytic meningitis do not usually need treatment other than resting at home.

Patients with paralytic polio may be placed on a respirator to help them breathe, particularly if they are diagnosed with bulbar polio. Other treatments include painkillers and hot packs for muscle aches, physical therapy to restore muscle strength, and occupational or speech therapy as needed. Braces or special shoes may be recommended for some patients. A few patients may undergo surgery to restore limb function.


The overall prognosis for recovery from an acute attack of paralytic polio is generally good. Mortality is about 5–10 percent, mostly in elderly and very young patients. Half the patients with spinal polio recover fully; 25 percent have mild disabilities; and the remaining 25 percent are left with severe disabilities. Most patients recover from breathing problems, and only a small percentage of patients need long-term treatment on a respirator. Patients with muscle paralysis typically recover about 60 percent of their strength in the first three to four months of treatment.

Some patients who have recovered from paralytic polio develop a disorder called post-polio syndrome (PPS) between ten and forty years after the initial infection. PPS is not a reinfection, although its cause is not completely understood. PPS is marked by:

  • Muscular weakness
  • Fatigue
  • Being easily exhausted after even small amounts of activity
  • Joint pain
  • Sleep disorders
  • Difficulty breathing or swallowing
  • Inability to tolerate cold temperatures

PPS is treated with rest and such supportive measures as powered wheelchairs, pain relievers, and medications to help the patient sleep. Patients are also encouraged to simplify their work habits and take frequent rest breaks.


Polio can easily be prevented by administration of either the Salk vaccine, which contains an inactivated poliovirus, or the Sabin oral vaccine, which contains a weakened live virus. The Salk vaccine is usually given in two doses four to eight weeks apart followed by a third dose six to twelve months after the second dose. It is the only polio vaccine that is given to people with weakened immune systems.

The Sabin vaccine is given in a single dose by mouth. It is not routinely given to people with weakened immune systems.

The Future

There is hope that polio will follow smallpox as a disease that humankind has completely wiped out. In 1994 both North and South America were

declared polio-free, followed by Australia, Japan, China, and other countries around the Pacific Ocean in 2000, and Europe in 2002. As of 2008, however, there were still four countries in the world where polio was endemic: Nigeria, India, Pakistan, and Afghanistan. Public health doctors hope that the vaccination campaigns that are underway in these countries will clear them of the disease in the next few years.

SEE ALSO Chronic fatigue syndrome; Influenza; Sleep apnea


Asymptomatic: Having no symptoms.

Endemic: A term applied to a disease that maintains itself in a particular area without reinforcement from outside sources of infection.

Meningitis: Inflammation of the membranes that cover the brain and spinal cord.

Motor neuron: A type of cell in the central nervous system that controls the movement of muscles either directly or indirectly.

For more information


Krasner, Robert I. Twentieth-Century Microbe Hunters. Sudbury, MA: Jones and Bartlett Publishers, 2008. This is a collection of short biographies of the discoverers of the polio vaccines as well as other eminent doctors.

Krohn, Katherine. Jonas Salk and the Polio Vaccine. Mankato, MN: Capstone Press, 2007.

Presley, Gary. Seven Wheelchairs: A Life beyond Polio. Iowa City: University of Iowa Press, 2008.


Zaracostas, John. “Failed Vaccine Campaigns May Lead to Polio Resurgence, Experts Warn.” British Medical Journal 333 (October 21, 2006): 7573. Available online at (accessed May 27, 2008). This is a news item about the reappearance of polio in parts of Africa and Asia.


American Experience: The Presidents: FDR. Polio Strikes. Available online at (accessed May 28, 2006). This is a twelve-minute video about President Franklin Roosevelt's attack of polio in 1921.

Centers for Disease Control and Prevention (CDC). Poliomyelitis. Available online in PDF format at (updated 2006; accessed May 27, 2008). This fourteen-page

pamphlet includes a historical overview of the disease as well as medical information.

Rx for Survival: Deadly Diseases. Polio. Available online at (accessed May 28, 2008). The page contains links to two videos on the campaign to eradicate polio in India; the videos take about fifteen minutes to play.

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