Doctors at the US Centers for Disease Control and Prevention (CDC) in the US say they have reached a "major milestone" in their quest to eradicate polio.
US CDC experts think a second of the three forms of poliovirus has been eliminated after mass vaccination campaigns. Wild poliovirus type 3 has not been found for more than two years, while type 2 was eradicated in 1999.
Polio, also referred to as poliomyelitis, is a highly infectious viral disease that generally targets young children, causing symptoms including pain, muscle weakness and fatigue. It causes paralysis in one out of 200 people and in the worst cases some children die when the muscles involved in breathing stop working.
Polio has been wiped out in the UK because of a comprehensive vaccination campaign delivered to all children in the country in a five-stage programme, beginning at two months of age, through to preschool and as teenagers.
Dr Stephen Cochi, a senior adviser at the CDC's centre for global health, said: "We may have eradicated a second of three: that's a major milestone".
But he told the BBC there was still a long way to go before type 1 polio is eradicated.
"It's the most prickly one. For reasons that are unclear, this is the most common cause of polio outbreaks and the most frequent cause of paralytic polio."
In Nigeria cases have fallen to six so far this year from 53 in 2013. But the number of cases recorded in Pakistan, where the Taliban stopped polio vaccination programs in some areas of the country, have leap from 59 last year to 236 and counting in 2014, and there is a risk that polio could spread to neighbouring countries.
Dr Cochi added: "The good news is now those children are accessible in refugee camps or other parts of the country so they are getting vaccinated. But the bad news is the poliovirus has spread all over the country and there have been cases from Karachi and Punjab province."
Prof Walt Orenstein, from the vaccine centre at Emory University in the US, told the BBC: "Type 3 appears to be gone – I think it is overwhelmingly likely that we are there, but it's too soon to say we're definitely there. It's not a total victory, but it is very promising."
But he warned: "Pakistan is a major concern, about 85% of wild type 1 poliovirus this year has been in Pakistan, but in Nigeria there is real hope we can get rid of type 1 even by the end of this year."