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Eight years before the Great Depression and two decades before the United States entered World War II, Franklin Delano Roosevelt's mettle was tested by another crisis. In 1921, the future president lost the use of his legs after contracting poliomyelitis (polio), a contagious viral illness that in its most severe form causes paralysis, difficulty breathing and sometimes death.

His wife, Eleanor, would later remark that "the thing that took (the) most courage in his life was his mastery and his meeting of polio." The disease challenged many other Americans as well. During the first half of the 20th century, no illness inspired more dread and outright panic than did polio, sometimes called infantile paralysis, which struck the nation every summer and fall with increasingly virulent epidemics. In 1952, when the disease was at its peak, 3,000 people died.

By the mid-1950s, mass immunizations began to slow polio's spread, and in 1979 the last case of wild polio — that is, polio not caused by a vaccine — occurred in the United States. Today, despite a concerted global eradication campaign, polio continues to afflict children and adults in other parts of the world, especially west and central Africa and India. Precautions are advisable before traveling to certain parts of the world.

Despite polio's ability to cause paralysis and death, the vast majority of people who contract the virus don't become sick and are never aware they've been infected. Yet even without any sign of illness, they can still shed the virus intermittently in their stool for several weeks and occasionally for months. In areas with poor sanitation, a single person harboring the virus can potentially infect hundreds of others before the first case of polio that causes paralysis appears. What's more, immune children and adults also can carry the virus, infecting others and sometimes contaminating sewage systems.

Nonparalytic polio
A small number of people develop nonparalytic polio — a type of polio that doesn't lead to paralysis (abortive poliomyelitis). This usually causes the same mild, flu-like symptoms that are typical of other viral illnesses: sore throat, fever, nausea, vomiting and constipation or diarrhea. Most people recover from abortive polio in less than a week.

About 5 percent to 10 percent of infected people develop nonparalytic aseptic meningitis, a viral infection of the outer covering (meninges) of the brain. Signs and symptoms, which generally last two to 10 days, include:

• Fever
• Headache
• Vomiting
• Diarrhea
• Fatigue
• Back pain or stiffness
• Neck pain or stiffness
• Pain or stiffness in the arms or legs
• Muscle spasms or tenderness

Paralytic polio
Fewer than 1 percent of people with polio infection develop paralytic polio, the most serious form of the disease. Paralytic polio often begins with a fever. Five to seven days later, other signs and symptoms appear, including:

• Headache
• Neck and back stiffness
• Constipation
• Unusual sensations or increased sensitivity to touch

Acute flaccid paralysis, which causes limbs to appear loose and floppy, often comes on suddenly and usually affects only one side or is worse on one side than on the other.

Polio has historically been divided into several types, depending on which part of the body is affected. These classifications aren't rigid, however, and there is overlap among the different forms. In the past, particularly, distinctions among types of polio may have varied with the method and time of diagnosis.

• Spinal polio. The most common form of paralytic polio, this attacks certain nerve cells (motor neurons) in your spinal cord and may cause paralysis of the muscles that control breathing as well as the muscles in your arms and legs. The muscles affected and the extent of damage depend on the part of the spinal cord and the number of neurons involved. Although paralysis can occur in any combination of limbs — for instance, both legs and one arm — children younger than age 5 are most likely to become paralyzed in a single leg, whereas paralysis of both arms and legs is more common in adults. Sometimes the neurons are only damaged, in which case you may recover some degree of muscle function. When the neurons are completely destroyed, the paralysis is irreversible, although you still retain your sense of feeling, which isn't the case in many spinal cord injuries.
• Bulbar polio. In this severe type of polio, the virus affects the motor neurons in your brainstem, where the centers of the cranial nerves are located. These nerves are involved in your ability to see, hear, smell, taste and swallow. They also affect the movement of muscles in your face and send signals to your heart, intestine and lungs. Bulbar polio can interfere with any of these functions but is especially likely to affect your ability to breathe, speak and swallow and can be fatal without respiratory support.
• Bulbospinal polio. A combination of both bulbar and spinal paralytic polio, this form can lead to paralysis of your arms and legs and may also affect breathing, swallowing and heart function.

Post-polio syndrome
Affecting people who have recovered from polio, post-polio syndrome is a cluster of disabling signs and symptoms that appear decades — between 10 and 40 years — after the initial illness. Common signs and symptoms include:

• New muscle weakness in limbs that may have been originally affected or in limbs that didn't seem to have been affected at the time
• General fatigue and exhaustion with minimal activity
• Muscle and joint pain
• Breathing or swallowing problems
• Sleep-related breathing disorders, such as sleep apnea
• Decreased tolerance of cold temperatures

Some viruses, including those that cause SARS, West Nile, monkeypox and, most recently, avian flu, are transmitted to humans through animal hosts. But the poliovirus resides only in humans and enters the environment in the feces of someone who is infected. The virus spreads primarily through the fecal-oral route, especially in areas where sanitation systems are inadequate.

The poliovirus also can be transmitted through contaminated water and food — there's some evidence that flies may spread the virus to food — or through direct contact. It's so contagious that anyone living with an infected person is likely to become infected too. Although people carrying the virus are most contagious seven to 10 days before and after signs and symptoms appear, they can spread the virus for weeks in their feces.

Once the poliovirus invades your body, it multiplies in your throat and intestinal tract and then travels to your central nervous system through your blood and lymph. As it moves along your nerve fibers, the virus damages or destroys the motor neurons that carry messages between your brain and your muscles.

To compensate for the missing neurons, the remaining nerve cells sprout new branching fibers (axons). Depending on how many neurons have been damaged, this regeneration may allow you to regain some or all of your muscle function, but it also places added stress on the nerve cell body, which has to nourish the additional fibers. Over time, this stress may be more than the neuron can handle, leading to the gradual deterioration of the new fibers and eventually to the neuron itself. Researchers theorize that this may lead to the recurrence of signs and symptoms that occur with post-polio syndrome.

You're at greatest risk of polio if you haven't been immunized against the disease. In areas with poor sanitation and sporadic or nonexistent immunization programs, the most vulnerable members of the population — pregnant women and the very old and very young — are especially susceptible. Polio, which once affected infants and children almost exclusively, now targets people of all ages.

These factors also increase your risk:

• Travel to an area where polio is endemic or that has recently experienced an outbreak.
• Living with or caring for someone who may be shedding wild poliovirus.
• Handling laboratory specimens that contain polioviruses.
• A compromised immune system, such as occurs with HIV infection.
• Trauma to your mouth, nose or throat such as dental surgery or a tonsillectomy.
• Extreme stress or strenuous physical activity after being exposed to the virus. Both emotional and physical stress can depress your immune system.

Paralytic polio
It's not known why some people develop paralytic polio and others don't. But many of the same factors that put you at risk of polio also make it more likely that you'll develop the paralytic form if you're infected. These include:

• A compromised immune system
• Pregnancy
• Mouth or throat surgery
• Injury or strenuous physical exercise after exposure to the virus

Post-polio syndrome
Although post-polio syndrome still isn't well understood, certain factors may increase your chances of developing this complex of symptoms decades after your initial infection:

• Severe polio infection. The more severe the initial infection, the more likely that you'll have signs and symptoms of post-polio syndrome.
• Age at onset of initial illness. If you acquired polio as an adolescent or adult, rather than as a young child, your chances of developing post-polio syndrome increase.
• Recovery. Paradoxically, the greater your recovery after acute polio, the more likely it is that you'll develop post-polio syndrome. This may be because greater recovery places additional stress on motor neurons.
• Physical activity. Engaging in physical activity to the point of exhaustion or fatigue may overwork already stressed motor neurons.

Wild polio has been eliminated from most parts of the world. Africa, which had the majority of cases in 2004, India, Pakistan and Afghanistan are notable exceptions. In the United States, the last case of wild polio occurred in 1979. Between 1980 and 1999, however, 144 people developed paralytic polio from exposure to the live virus in the oral polio vaccine. For that reason, only vaccine containing inactivated virus has been used in the United States since 2000.

Call your doctor if:

• Your child hasn't completed the series of vaccinations that protect against polio
• Your child experiences an allergic reaction after receiving the polio vaccine
• Your child has problems other than a mild redness or soreness at the injection site
• You have questions about adult vaccination or other concerns about polio immunization
• You had polio years ago and are now experiencing unexplained weakness and fatigue
• You're planning travel to parts of the world where polio still occurs

Doctors can often recognize polio by symptoms such as neck and back stiffness, abnormal reflexes and difficulty swallowing and breathing. To confirm the diagnosis, a sample of throat secretions, stool or cerebrospinal fluid — a colorless fluid that bathes the brain and spinal cord — is grown on a special medium and checked for the presence of the poliovirus.

Paralytic polio can lead to temporary or permanent muscle paralysis, disability and deformities of the hips, ankles and feet. Although many deformities can be corrected with surgery and physical therapy, these treatments often aren't options in the developing nations where polio is endemic. As a result, children who survive polio may spend their lives with grave disabilities.

Other complications of polio involve the lungs, kidneys and heart and may include:

• Pulmonary edema. Potentially life-threatening, pulmonary edema occurs when increased pressure in the blood vessels in your lungs forces fluid into the air sacs, filling your lungs with fluid and preventing them from absorbing oxygen.

• Aspiration pneumonia. This inflammation of the lungs and bronchial tubes results when you inhale foreign material — usually stomach contents — into your lungs. Although many factors can cause aspiration pneumonia, it's most often a complication of polio when the disease affects the muscles that control swallowing.
• Urinary tract infections. These infections, which begin when bacteria enter your bladder through the urinary tract, can permanently damage your kidneys if they're not treated promptly.

• Kidney stones. Kidney stones usually form when your urine becomes too concentrated, causing minerals and other substances in urine to form crystals on the inner surfaces of your kidneys. Over time, these crystals may combine to form a small, hard mass, or stone. If a stone stays inside one of your kidneys, it usually doesn't cause a problem unless it becomes so large it blocks the flow of urine. This can cause pressure and pain, along with the risk of severe kidney damage, bleeding and infection. Smaller stones may partially block the thin tubes that connect each kidney to your bladder or the outlet from the bladder itself. These stones may cause ongoing urinary tract infection or kidney damage.

• Intestinal obstruction (paralytic ileus). This is a partial or complete blockage of your bowel that prevents food from moving through the intestinal tract. Minor blockages may cause bloating and a distended abdomen, but more serious obstructions can lead to life-threatening complications.
• Myocarditis. In this condition, the thick muscular layer of your heart (myocardium) becomes inflamed, leading to chest pain, an abnormal heartbeat or congestive heart failure, which occurs when your heart can't pump enough blood to meet your body's needs. Myocarditis can also cause blood clots to form, greatly increasing your risk of stroke.

• Cor pulmonale. The result of prolonged high blood pressure in the veins or arteries in your lungs, cor pulmonale occurs when the right side of your heart can't pump hard enough to compete with the lungs' increased blood pressure.

In 1916, a large polio outbreak in the United States — and ineffectual efforts to contain it — spurred researchers to find new and better treatments for the disease. What resulted were two of the more notorious developments in polio's long and difficult history: the iron lung and a serum derived from the blood of human and animal polio survivors. Although the serum had no known benefit, it was widely administered for nearly two decades.

The iron lung was another matter. It did indeed save lives, helping people with paralyzed chest muscles breathe. But the price exacted was enormous. About the size and weight of a small car, the iron lung is a sealed chamber with an electrically driven bellows that regulates breathing. People with polio were encased in the metal chambers for months, years and sometimes for life.

Other therapies
A number of other therapies, including a "head sling" and rigid braces and body casts that caused muscle atrophy, were tried on children in an effort to prevent deformities. Doctors continued using these treatments, which often caused psychological and physical harm, until the 1940s, when an Australian army nurse, Elizabeth Kenny, proposed treating afflicted limbs with massage, exercise and warm, moist heat.

Today, Kenny's therapy, which took years to gain acceptance, is still used to stimulate muscles. In combination with antispasmodic medications that reduce muscular contractions it can improve mobility, although it can't reverse permanent paralysis.

Because no cure for polio exists, the focus is on increasing comfort, speeding recovery and preventing complications. Supportive treatments include:

• Antibiotics for infections
• Analgesics for pain
• Portable ventilators for breathing problems
• Moderate exercise
• A nutritious diet

Although improved sanitation on the public level and fastidious hygiene on the personal may help reduce the spread of polio, the only real way to prevent the disease is with the polio vaccine.

However, efforts to banish polio worldwide face major obstacles. Some communities in parts of the world, concerned about the safety of the vaccine, have resisted immunizations. In other areas, war and civil disorder prevent health workers from reaching vulnerable populations. In addition, travelers in under-vaccinated countries may unwittingly carry the virus into previously polio-free zones.

Vaccine history: A heated rivalry
Two researchers, Jonas Salk and Albert Sabin, were contentious rivals in the effort to produce the first and safest vaccine. Salk got his vaccine, which used an inactivated form of the virus, into production first, and it was tested in a field trial in 1954. The trial of the Salk vaccine — poliovirus vaccine inactivated (IPV) — was unprecedented, both in scope and sheer audacity: an unproved and potentially dangerous vaccine was given to 650,000 young children, the fear of polio apparently greater than fear of the drug.

Sabin was so convinced that Salk's approach was wrong that he tried, unsuccessfully, to block the trial. But Salk's drug was declared safe and effective at the conclusion of the tests, and mass public immunizations began almost immediately. IPV had its drawbacks, however. It wasn't 100 percent effective, it had to be given in a series of three injections and there was concern that it didn't provide lifelong immunity.

By the time Sabin's vaccine was licensed in 1962, polio cases and the hysteria surrounding the disease had both dropped dramatically, and the public was ready for something different. Sabin's oral polio vaccine (OPV) fit the bill since it conferred lifelong immunity and was simple to administer because it was given by mouth.

But the Sabin vaccine used a weakened form of live virus, and though that provided the added benefit of transferring immunity to unvaccinated people in the community, it also caused an average of eight cases a year of actual polio. For a time, doctors used a combination of the two vaccines, but the Sabin vaccine was discontinued in the United States in 2000, although it's still used in the global eradication campaign.

The polio vaccine today
Currently, most children in the United States receive four doses of IPV at the following ages:

• 2 months
• 4 months
• Between 6 and 18 months
• A booster shot, between 4 and 6 years of age when children are just entering school

In some states, the fourth shot isn't required if children receive the third shot on or after their fourth birthday.

IPV is 90 percent effective after two shots and 99 percent effective after three shots. It can't cause polio and is safe for children with weakened immune systems, although it's not certain just how protective the vaccine may be in cases of severe immune deficiency. The most common side effects are swelling or redness at the injection site.

IPV can cause serious allergic reactions in some children and adults. Because the vaccine contains trace amounts of the antibiotics streptomycin, polymyxin B and neomycin, it shouldn't be given to anyone who is sensitive to these medications.

Signs and symptoms of an allergic reaction usually occur within minutes to a few hours after the shot and may include:

• High fever
• Difficulty breathing
• Weakness
• Hoarseness or wheezing
• Rapid heart rate
• Hives
• Dizziness
• Unusual paleness
• Swelling of the throat

If your child experiences an allergic reaction following any shot, get medical help immediately.

Fewer shots for your child
The polio vaccine is normally given in conjunction with other vaccinations, including diphtheria, tetanus and acellular pertussis (DtaP), hepatitis B-Haemophilus influenzae type b (HBV-Hib) and pneumococcal conjugate vaccine (PCV). But your child may not need to receive all these injections separately.

In December 2002, the Food and Drug Administration approved a new combination vaccine called Pediarix. The vaccine reduces the number of injections given during the first two years of life by combining several required vaccines — DTaP, hepatitis B and polio — into a single vaccine.

Pediarix has drawbacks, however. It's not available in all areas, and even when it's available, your doctor may not use it or it may not be covered by your insurance plan. In addition, Pediarix can cause a number of side effects — some of them serious — and it's not approved for use as a booster shot, the fourth in the series. If you have questions, talk to your doctor.

Adult vaccination
In the United States, adults aren't routinely vaccinated against polio because most are already immune and the chances of contracting wild polio are minimal. But vaccinations are recommended for certain high-risk adults, including those who are traveling to parts of the world where polio still occurs — primarily west and central Africa, India, Pakistan and Afghanistan — or those who care for people who may be excreting wild polioviruses.

If you fall into one of these categories, you would normally receive two doses of IPV, administered at four to eight week intervals, and a third dose six to 12 months after the second. If you're traveling and don't have time to get all three vaccinations, you may receive one or two — the second at least four weeks after the first — and then complete the series later.