The Centers for Disease Control (CDC) has announced that a “major milestone” in the battle to eliminate polio globally has been reached.

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 detected for more than two years. Type 2 was eradicated in 1999.

Experts said the world was “closer than ever” to defeating polio but the situation in Pakistan was worrying.

Polio is highly infectious and causes paralysis in up to one in 200 people. Some children die when the muscles that help them breathe stop working.

However, there has been huge progress in eliminating the disease. Cases have fallen from 350,000 in 1988 to 416 in 2013.

According to the CDC report, the last case of type 3 poliovirus was detected in Pakistan in November 2012.

“We may have eradicated a second of three; that’s a major milestone,” said Dr. Stephen Cochi, a senior adviser at the CDC’s Centre for Global Health.

However, a formal process – involving the Polio Global Certification Commission – is required before type 3 can be officially declared eradicated. That will not take place for at least another year.

Type 1 remains endemic in three countries – Pakistan, Afghanistan and Nigeria.

There has been progress in Nigeria, where cases have fallen to six so far this year from 53 in 2013.

Cases have leap from 59 last year to 236 and counting in 2014.

The Pakistani Taliban stopped polio vaccination programs in some tribal areas of the country for about two years.

Since the summer there has been a mass exodus of people from the region after military operations by Pakistan’s army.

It means there is a serious risk of polio spreading to other countries.

Polio virus traveled from Pakistan to Syria in 2013.