Millions of ordinary house cats are carrying an illness which could infect humans, causing persistent diarrhea, nausea and even more severe issues in extreme cases. 

A new study by US veterinary researchers found that roughly one in 15 felines are infected with a tiny gut parasite called Giardia, which can cause a condition commonly known as 'beaver fever.'  

The parasite can spread and get you sick when people ingest microscopic cysts of Giardia hiding in feces, contaminated water and on dirty hands or surfaces.

In the US, more than a million people get beaver fever every year, typically suffering from diarrhea up to five times a day, stomach cramps, nausea and dehydration lasting for two to six weeks.

In some serious cases, beaver fever can drag on for years, causing joint pain, symptoms of irritable bowel syndrome, weight loss and issues absorbing nutrients from food.

A team from the commercial veterinary lab Antech Diagnostics and other pet care facilities tested over 157,000 stool samples from cats in the US and Canada, finding over six percent were positive for Giardia.

Among the different strains of Giardia infecting house cats throughout the US, nearly one in five had a version of the parasite which can jump to humans and cause beaver fever. There are over 70 million household cats in the US.

Dr Stevie Aitken, an emergency veterinarian, said: 'If your cat is diagnosed with Giardia, thoroughly cleaning all litter boxes, water and food bowls is imperative.'

Beaver fever got its name in the 1970s, when outbreaks of the illness were traced back to untreated water in areas where beavers lived.

Beavers likely passed the parasitic cysts into local streams and lakes, which campers and hikers drank from, consuming this hardy and resilient form of Giardia, which can survive in cool and moist environments like soil and water for months.

Among pet cats, contamination can occur by touching a used litter box, soiled fur, or a contaminated surface and then touching your mouth.

Someone could also get infected with the parasite by drinking or coming into contact with water or food that was contaminated with Giardia-positive feces from a cat. Only a few cysts are needed to trigger a case of beaver fever.

The study published in PLOS One examined 157,616 cat stool samples from 49 US states, Washington DC and five Canadian provinces using a sensitive DNA test to look for Giardia.

Out of that group, 9,962 tested positive for the parasite. These cats were living throughout the entire US, with the largest concentrations in the Northeast, including in Maryland, Pennsylvania, New Jersey, New York and Massachusetts.

Among the cats testing positive for Giardia, 18.5 percent carried a strain of the parasite known as 'assemblages A and B,' the types that can easily jump to people and cause beaver fever.

The rest of the positive cats mostly carried a cat-only type known as assemblage F, which only makes people sick on rare occasions.

Based on the study's findings, the results suggest that roughly five million cats throughout the US, and in many populated cities, may be carrying some form of Giardia.

Out of those pets, as many as a million could have a strain that easily passes to humans and could cause a lingering illness from touching contaminated surfaces.

'Clean any areas where accidents may have occurred and keep them separated from young children or immunocompromised individuals until treatment and re-testing is complete,' Dr Aitken told ABC News.

Among the house cats studied, the parasite was more commonly found among younger cats and kittens, and slightly more often among males.

Senior cats have the lowest chance of carrying the parasite, and researchers were surprised to find that intact cats were less likely to have Giardia than those who were spayed or neutered.

The researchers warned that Americans needed to be extra cautious around their pets now as the seasons change from summer to fall.

The study revealed that cases of the parasite were at their lowest in the summer and rose across fall, winter and spring.