Polio largely vanished thanks to vaccines. So why is it now back in more countries?

Polio, a potentially disabling virus that's long been forgotten in many parts of the world, is now circulating in parts of the U.S. and U.K., on the heels of an outbreak in Israel — prompting health officials to launch vaccination campaigns to ensure the public is protected.

Even in Canada, a country free of polio for the last two decades, medical experts say it's a wake-up call that the virus still poses a threat to anyone who remains unvaccinated, given polio's ability to spread through global travel networks and wastewater systems.

On Wednesday, British health authorities announced they will offer a polio booster dose to children aged one to nine in London, after finding evidence the virus has been spreading in multiple regions of the capital. Britain's Health Security Agency said polio virus samples were found in sewage water from eight boroughs of London, but there were no confirmed infections.

Still, the agency's analysis showed transmission has likely "gone beyond a close network of a few individuals."

"We know the areas in London where the poliovirus is being transmitted have some of the lowest vaccination rates," said Dr. Vanessa Saliba, a consultant epidemiologist at the U.K. Health Security Agency. 

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The agency said it was working closely with health authorities in the U.S. and Israel, as well as the World Health Organization, to investigate the links between polio viruses detected in those two countries. 

In July, Israel announced a recent outbreak of polio infections appeared to be under control, after multiple people became infected, including a Jerusalem girl who was paralyzed and now requires rehabilitation, according to the Jerusalem Post.

More recently, in the state of New York, one unvaccinated young adult suffered paralysis after a polio infection in Rockland County — an area known for low vaccination rates — which marked the first case reported in the U.S. in nearly a decade. 

Vaccination campaigns are now underway as samples of the virus were also detected in the wastewater of both Rockland and another county, just north of New York City. Officials also say hundreds more people may already be infected.

"The scope may be even much larger than we can even fathom or imagine," said New York-based immunologist Dr. Purvi Parikh.

"Because vaccines have become the victim of their own success, we don't see it — so we may not even realize what the spread of polio is, because for the majority of us who are vaccinated, it doesn't really [affect] us."

Before mass vaccination campaigns largely wiped out polio from circulation in higher-income countries like Canada between the 1950s and early 1990s, the virus was known for sparking outbreaks and striking children, causing paralysis or death in some cases.

Even now, war, poverty, and social unrest make it difficult to achieve eradication in many countries around the world. Outbreaks remain common in Afghanistan, Pakistan, and parts of Africa.