Polio outbreaks in New York and London are spreading, in part, through human feces and bad sanitation habits.

One New York man was found to have been infected with polio earlier this summer, the first American case in years and he ended up temporarily paralyzed, according to ABC News. Authorities then discovered the virus spreading throughout the wastewater systems both in London and in various parts of New York, indicating that more cases are going undetected.

The New York man suffered from vaccine-derived poliovirus. There are two types of polio vaccines in common use today: an injection which uses a dead version of the virus, and an oral vaccine that uses a live, but weakened, version of the virus, according to the Polio Global Eradication Initiative.

#HCPs: Polio has been detected in New York state. While there is no cure for #polio, it can be prevented with a safe and effective vaccine. Here are five key things you should know about polio: https://t.co/I8qgGdHk3U pic.twitter.com/uipZ9FOgaZ

— CDC (@CDCgov) August 29, 2022

Both vaccines offer substantial protection from catching polio or suffering from serious symptoms. There are now only a few dozen wild poliovirus cases in a typical year worldwide, with the virus having been nearly eradicated from the United States and Europe. There are slightly more cases, typically numbering in the hundreds per year, that occur from use of the oral vaccine due to the live virus inside, according to the Polio Global Eradication Initiative.

The oral vaccine, in somewhere around one in two to three million recipients, can cause paralysis from the virus, according to the Children’s Hospital of Philadelphia. That risk isn’t present in the inactivated injected vaccine, which is why the U.S. and Europe discontinued use of the oral vaccine in favor of the shot years ago. However, in much of the developing world, the cheaper, easier to produce and administer oral vaccine is still the predominantly used option.

After receiving the oral vaccine, the virus sheds in the feces of an individual, sometimes for months or even years. It’s incredibly rare that that shedding leads to active cases of the virus, but there is a drawback. According to medical professionals, if the virus continues to spread amongst unvaccinated people as a result of this shedding, it can lead to the virus evolving into a strengthened version which can cause paralysis, according to The Economist. That process typically takes about one year.

That’s why health officials are now surveilling wastewater in New York, London and elsewhere for the virus. The detection of polio in the wastewater means that people are shedding the virus in their stool. It’s believed the New York case that resulted in paralysis was the result of transmission that initiated overseas where the oral vaccine is in use, according to the outlet.

While polio can spread through close contact and respiratory droplets, it can also spread through contact with feces. That’s why it’s more common to see polio outbreaks in countries with inferior sanitation systems. (RELATED: CNN Reporter Appears To Mock McConnell’s Polio Struggles [VIDEO])

“It spreads through contact with the stool (poop) of an infected person or droplets from a sneeze or cough. If you get stool or droplets from an infected person on your hands and you touch your mouth, you can get infected,” according to the Centers for Disease Control and Prevention (CDC). “Also, if your child puts objects, like toys, that have stool or droplets on them into their mouth, they can get infected.”

“The polio virus enters the body through the mouth, usually from hands contaminated with the stool of an infected person,” according to the New York Department of Health.

The inactivated polio vaccine used in the U.S. has a virtual 100% success rate at preventing paralysis from polio. But just under 94% of kindergarten-aged kids nationwide are vaccinated against the virus and the coronavirus pandemic has led to a resurgence in vaccine skepticism, according to the CDC.

The inactivated vaccine doesn’t necessarily prevent transmission of the virus, so it’s possible that vaccinated individuals who don’t experience symptoms are now passing it on to the unvaccinated, creating risk of more paralysis cases. New York wastewater samples show higher rates of shedding in feces in areas with lower vaccination rates, and the man who became infected in New York was unvaccinated.

“We probably could never have gotten on top of polio in the developing world without the (oral polio vaccine), but this is the price we’re now paying,” Dr. Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, told CBS News. “The only way we are going to eliminate polio is to eliminate the use of the oral vaccine.”