The World Health Assembly resolved to eradicate polio globally by 2000, but a state of emergency over polio declared by New York on Friday has sparked questions about whether it was truly eradicated.  

New York Governor Kathy Hochul declared the state of emergency amid evidence that the virus is spreading in communities, as polio was detected in sewage samples from four counties near the city.  

The declaration came over a month after an unvaccinated adult caught polio in Rockland County, north of New York City and suffered from paralysis. It's the first known polio infection in the U.S. in nearly a decade. 

Polio is a disease caused by a virus that lives in the human throat and intestinal tract. It spreads from person to person by exposure to infected human stool. The virus often spreads unnoticed because 70 percent of people infected don't show any visible symptoms. 

One in 100 people infected become permanently paralyzed. Among people suffering from paralysis, the fatality rate is between 2 and to 10 percent. The virus primarily affects children aged three and younger, and there is no known cure. 

The first major documented polio outbreak in the U.S. occurred in Rutland County, Vermont in 1894, when 18 deaths and 132 cases of permanent paralysis were reported. However, it wasn't until 1905 that polio was discovered to be contagious. 

In 1916, U.S. health officials announced a polio outbreak centered in Brooklyn, New York, which took the lives of over 6,000 people, leaving thousands more paralyzed. In 1952, polio cases surged to a record high, and the worst outbreak focused public awareness on the need for a vaccine. 

Polio was largely eradicated from the U.S. by a vaccine that became available in 1955. The doses provide nearly 100 percent immunity. 

Since the World Health Assembly resolved to eradicate polio globally, substantial progress has been reported from six World Health Organization (WHO) regions. The number of paralytic polio cases decreased by 99 percent from 34,617 in 1988 to 496 in 2001. 

There are three strains of wild polio, two of which have officially been certified as globally eradicated. But wild polio Type 1 still affects a few countries. 649 paralytic polio cases were reported globally in 2021.

The best course of preventing polio is to get vaccinated, according to WHO officials. There are two types of vaccines: an inactivated poliovirus vaccine (IPV) given as an injection in the leg or arm, and an oral poliovirus vaccine (OPV). 

While the vaccine is very effective at preventing the disease, it does not block transmission of the virus. In rare cases, the virus used in the OPV can mutate, become virulent and spread to others, which is called vaccine-derived polio. 

However, the WHO said children are far more at risk from polio than any side effects from the vaccine. It takes just a single case to put the public at risk, with people who never had a polio vaccine most vulnerable to the infection.

The polio vaccination rate is alarmingly low in some New York counties, with the rate at 60 percent in Rockland, 58 percent in Orange, 62 percent in Sullivan and 79 percent in Nassau - the districts where polio has been detected - according to New York officials.  

It is possible that hundreds of people statewide have gotten the virus and do not know it, according to health officials. Friday's emergency declaration aims to boost vaccination rates from the current statewide average of about 79 percent to above 90 percent.