One strain of the virus that causes polio has likely been eradicated worldwide, according to a new report.

For two years now, there have been no new reports of polio caused by this strain, called poliovirus type 3, according to researchers from World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC).

The possible eradication of the poliovirus type 3 would be a "historic milestone" for global polio eradication efforts, the researchers said in their report released today (Nov. 13).

The world is not yet rid of polio. There are three strains of the virus, and type 1 is still actively being transmitted in three countries: Nigeria, Pakistan and Afghanistan. Transmission of poliovirus type 2 has been stopped since 1999. [5 ScariestDisease Outbreaks of the Past Century]

Poliovirus causes polio, or poliomyelitis, as the condition is properly called. The disease is a highly contagious and incurable infection of the nervous system, but can be prevented by vaccination. In the 1980s, the virus killed or paralyzed about 350,000 people worldwide each year. But since the WHO's Global Polio Eradication Initiativewas launched in 1988, the number of cases has fallen by over 99 percent.

Still, because the type 1 poliovirus is spreading in three countries, the disease continues to pose a threat to any polio-free region in the world where vaccination levels are not sufficiently high, according to the World Health Organization.

Poliovirus type 1 is the most virulent of all strains of this virus, causing illness in 1 out of every 200 people it infects. Type 3 sickens about 1 in 2,000 people it infects.

The last case of polio caused by the type 3 strain of the virus was an 11-month-old infant in Nigeria, who became paralyzed on Nov. 10, 2012, according to WHO. Since then, no other cases have been reported, and the virus hasn't shown up in lab tests conducted by the organization's polio surveillance team. Still, continued surveillance will look at whether poliovirus type 3 is in fact fully stopped.

If confirmed, the successful eradication of poliovirus type 3 shows that implementing WHO's vaccination plans in the three remaining countries will alsostop the spread of poliovirus type 1, the researchers said.

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