Different Strains of the Polio Virus

Polio, or poliomyelitis, is a viral disease that is sometimes called infantile paralysis, because doctors once thought that only children got it. Polio is highly contagious and is spread by person to person contact. The virus enters the body orally and begins to multiply. Up to 95 percent of infected individuals may be completely asymptomatic, according to the Centers for Disease Control. Other patients may experience varying degrees of the illness, but the 1 percent who experience complete paralysis may never recover.

• Injection of Polio Vaccine

The Poliovirus Receptor -- CD-155

• Viruses that infect humans must enter the body's cells to multiply. Human cells have hundreds of receptors on their surfaces for various cell functions. The polio virus has evolved to attach to one receptor called CD-155. Once attached, the virus injects its genetic material into the cell where it produces hundreds of new viruses. It is postulated that those who do not get polio after exposure may not have the specific receptor on their cells. The polio virus has three strains, or serotypes, that all attach to the CD-155 receptors.

Type I -- the Brunhilde strain

• The Brunhilde strain was named for a female chimpanzee used in polio research at Johns Hopkins. The animal was injected with stool specimens from patients in the Baltimore area in 1939. The polio viruses isolated from the spinal cord of the chimp was classified as type 1, the Brunhilde strain.

Type II -- the Lansing strain

• The Lansing strain came from a patient who died from polio in Lansing, Michigan, in 1938. Type 2 is considered to be the easiest to eradicate and was believed to have been wiped out in 1999; however, the strain has resurfaced in Nigeria, one of the Third World countries still battling polio.

Type III -- the Leon strain

• Leon was an 11-year-old boy who died from paralytic polio in the Los Angeles epidemic of the 1930s. The type 3 strain was recovered from Leon's brain and spinal cord tissue. It is often the last to be eradicated in an area.

Polio Vaccines

• Jonas Salk developed the first polio vaccine from inactivated or killed polioviruses. Salk's vaccine is injected into the arm or thigh and may cause some level of discomfort. An oral vaccine developed by Albert Sabin is easier to administer and avoids painful injections. The oral vaccine uses live but weakened, or attenuated, viruses. Both vaccines use all three types of the polio virus, because even if a person develops immunity to type 1, he could still develop polio from the type 2 or type 3 strains. This is one reason for the re-emergence of type 2 in Nigeria; the vaccines in use only contained type 1 or type 3 viruses, so no immunity developed for type 2.

The Most Famous Polio Victim

• Franklin D. Roosevelt contracted polio in the summer of 1921. He fought back over the years with exercise and warm springs therapy, but he could never walk again unaided or without braces. Roosevelt bought a Georgia resort known for its warm mineral springs and formed the Georgia Warm Springs Foundation to help fellow polio victims. This evolved into the March of Dimes, which funded research that led to successful polio vaccines. The PBS documentary "The Polio Crusade" chronicles the search for a vaccine.