For the first time in nearly 40 years, polio has been found in London, health officials in the UK said, warning hospitals about an infectious disease that has not been documented in Britain for nearly 40 years.

Nevertheless, no polio cases have been reported in the UK. Rather, scientists have discovered the virus through an indirect route, sewage water.

The UK Health Security Agency (UKHSA) announced on Wednesday that it found several similar polioviruses in wastewater samples taken from a treatment plant in east London between February and May.

The UKHSA is working with other organizations, such as the World Health Organization, to urgently study the issue and understand the discovery’s magnitude.

Scientists are testing more sewage samples at other locations to try and find where the outbreak happened and whether it is spreading.

Epidemiologist Vanessa Saliba, who consults for the UKHSA, said, “We are urgently investigating to better understand the extent of this transmission.”

The viruses were found at the largest water treatment plant in the UK, the London Beckton Sewage Treatment Works.

Traces of polio can be found in the stools of people with polio or people who have been immunized with a live version of the virus. These traces ultimately end up in sewage wastewater.

This form of detection implies that the viruses have been spreading locally, the health agency said, adding the viruses’ genetic sequences indicate “there has been some spread between closely linked individuals in north and east London.”

Scientists believe that an infected individual carried the virus into London and then spread it to other people who were not immunized.

Virologist Angela Rasmussen, who studies polio at the University of Saskatchewan in Canada, said, “It sounds like the outbreak is very small. The outbreak could be within an extended family. Transmission would require a concentration of people who had not yet been vaccinated.”

UK public health officials plan to prevent the virus’ spread with targeted immunization. The agency encouraged anyone not fully immunized to schedule an appointment immediately.

“If you’re not up to date on your polio vaccine, now’s the time to go out and get up to date,” Rasmussen said.

Polio can, during rare occurrences, result in severe conditions such as paralysis.

Still, the danger to the general public is minor, the UKHSA said, because the vast majority of British citizens are immunized against the virus during childhood and thus safeguarded against disease. In the United States, polio immunization is a standard vaccination for babies and young children.

The United Kingdom declared it was polio-free in 2003, with the last case of wild polio in 1984.