A team of scientists captured 133 rats at various places in New York City’s subways, and used DNA sequencing to catalog the pathogens they were carrying.

They found an alarming number of disease causing bacteria, viruses, and protozoa, but what was most alarming was the viruses they found — 18 viruses that are entirely unknown to science.

Openings around service conduits like electricity and phone lines, and water pipes, provide easy access indoors and between apartments. Dryer vents and gaps around window-installed air conditioner units are other common entry points. Rodents can also crawl up through drain pipes not properly capped, as well as under exterior doors that don’t seal well.

The authors note that the new viruses have not been reported yet in humans, but it is possible that human infection with some of these viruses is already occurring. Because most viruses cause only mild symptoms, or symptoms very similar to other illnesses, many illnesses are misdiagnosed.

A few of the viruses stand out, including a new species similar to the hepatitis C virus, and Seoul hantavirus, a dangerous disease that causes excessive bleeding, and had never been found in New York before.

The study raises the question of how many more unknown viruses are lurking in rat populations around the country.