An outbreak of polio has been officially confirmed in Papua New Guinea. This is grim news, particularly as the sizable island was declared polio-free for 18 years.

According to various health authorities, the first case of the virus was found in a young boy back in April in Morobe province, and now others in the same community have also been infected. As noted by the Guardian, the vaccination coverage in Morobe province is low, and just under two-thirds of kids receive all three doses.

The country hasn’t got a great healthcare infrastructure and suffers from various hygiene and sanitation problems, which gives this virus – commonly transmitted via the fecal-oral route – a good chance at spreading further. It is, however, explained that it’s unlikely to spread to other countries because of both the nation’s isolation and the high level of immunization found in other parts of the world.

Polio, which only infects humans, is nearing eradication in the wild. Back in 1988, there were 350,000 cases worldwide in 125 different countries. In 2017, there were just 22, all thanks to a worldwide drive led by governments, health organizations, UNICEF, and private partners, including the Bill & Melinda Gates Foundation (BMGF), to eliminate the disease.

The rapid development of two cheap, highly effective multi-dose inoculations – an oral polio vaccine (OPV) and an intravenous, inactivated polio vaccine (IPV) – along with a global surveillance and response program means that the virus is hurtling toward the dustbin of history.

That’s a huge relief, of course: poliomyelitis, as it is known in full, has a range of negative health effects, but the worst is that in one out of 200 cases, it leads to a form of irreversible paralysis. Of those affected in this way, the World Health Organization (WHO) estimates that 5 to 10 percent die as their breathing muscles stop working.

There isn’t a cure once infection occurs; the only solution is to vaccinate people from it. The most vulnerable to infection are children under 5, which is why any signs of the disease in this demographic – or any, but this one especially – must be very quickly stamped out. This outbreak in Papua New Guinea is no exception to this rule.

This isn't the wild poliovirus making a reappearance, by the way. In this situation, it's an extremely uncommon type of vaccine-derived poliovirus (VDPV), and it's important to clarify what that is.

The OPV contains a weakened version of the real virus, which replicates in the gut and triggers an immune response. Like the wild virus, this is excreted through fecal matter for several weeks.

Sometimes, if the strain used in the OPV has mutated, it becomes a VDPV, which can infect others if immunisation rates in the area are low, or if people in the community have extremely weak immune system. In this case, the BMGF told IFLScience via email that the outbreak was caused by the former.

So far this year, there have only been 15 cases of polio. As BBC News explains, the day when zero cases are registered seems to be approaching, but the virus still infects people in Afghanistan and Pakistan.

“Failure to eradicate polio from these last remaining strongholds could result in as many as 200,000 new cases every year, within 10 years, all over the world,” the WHO explains.

Now, sadly, we can put Papua New Guinea back on the list. Let’s hope the outbreak there is quickly stamped out by medical teams, who have been on the ground since April issuing vaccinations to children still needing them.