High risk characteristics or behaviors for disease and for progression of disease include a lack of immunization against polio, larger inocula of poliovirus, travel to an area that has had a polio outbreak, pregnancy, very old or very young age, recent trauma to the mouth, nose or throat (such as tonsillectomy or dental surgery), and unusual stress or physical exertion after an exposure to poliovirus (since emotional and physical stress can weaken the immune system).

A polio virion is composed of a single-stranded positive-sense RNA genome of approximately 7,450 nucleotides enclosed in an icosahedral particle 27nm in diameter. The RNA is the infectious agent for poliomyelitis, while the viral capsid serves as a protective coat for the RNA. Polioviruses occur as three distinct serotypes, which differ structurally primarily in the loop regions of their viral coat proteins(VPs). The structures of Mahoney strain of type 1 and Sabin strain (which is used as the attenuated vaccine) of type 3 poliovirus have been solved by X-ray crystallographic analysis.

It is due to its most extreme clinical form: paralytic afflictions of the central nervous system, that the infection caused by poliovirus earns its name: poliomyelitis (polio= gray; myelos= marrow, spinal cord). There are three basic patterns of polio infection: subclinical, nonparalytic and paralytic, and in fact, poliomyelitis progresses to paralytic disease in less than 1 in 100 cases of infection. Although polio is most common in infants and young children, paralysis occurs at a higher incidence when infection occurs in older individuals.

In those fortunate individuals in whom the disease does not progress, poliomyelitis may be asymptomatic, or cause a slight fever, headache, or vomiting for 72 hours or less. The nonparalytic and paralytic forms of poliomyelitis, both of which affect the central nervous system, may occur after recovery from a subclinical infection. Nonparalytic polio is characterized by moderate fever, diarrhea, excessive tiredness, vomiting, muscle tenderness and spasm in any area of the body, and pain, all of which may last from one to two weeks. Paralytic polio is characterized by fever, headache, stiff neck and back, asymmetrical muscle weakness which has a rapid onset and may progress to paralysis, difficulty urinating and swallowing, muscle contractions or muscle spasms, difficulty breathing and abnormal sensations in an area. Paralytic poliomyelitis is further classified depending on the site of paralysis as spinal, bulbar, or spino-bulbar disease. Paralytic poliomyelitis is fatal in 2%-10% of all cases.

While there is no cure for polio, the goal of treatment is to control it symptoms while the virus runs its course. This can mean assisting a patient with breathing, treating with antibiotics for urinary tract infections and with analgesics to reduce headache, muscle pain and spasm. Physical therapy or orthopedic surgery may also be necessary to maximize the recovery of muscle strength and function.

In considering the spread of disease, it should be noted that patients are most infectious from seven to ten days before and after the onset of symptoms. However, patients are potentially contagious for as long as the virus is present in the throat and in the feces; the virus persists in the throat for approximately one week after the onset of illness and is excreted in the feces for several weeks or months thereafter. Furthermore, whether or not a patient develops lifelong immunity from infection depends upon which type of virus that a person contracts. Second attacks are rare and result from infection with a poliovirus of a different type than the first attack.

After an interval of thirty to forty years, 25%-40% of individuals who contract paralytic poliomyelitis in childhood will experience Post-Polio Syndrome. This disease entity consists of muscle pain and exacerbation of existing weaknesses or the development of new weaknesses or paralysis. Risk factors for post-polio syndrome include increasing length of time since acute poliovirus infection, presence of permanent residual impairment after recovery from the acute illness, and female sex. No cases of post-polio syndrome have been reported as a consequence of vaccine-associated paralytic polio.