ALBUQUERQUE, N.M. (AP) — A rodent-borne illness is suspected of causing an outbreak aboard a cruise ship that has killed three people and sickened others.

Studies indicate hantaviruses have been around for centuries, with outbreaks documented in Asia and Europe. In the Eastern Hemisphere, it has been linked with hemorrhagic fever and kidney failure. It wasn’t until the early 1990s that a previously unknown group of hantaviruses emerged in the southwestern United States as the cause of an acute respiratory disease now known as hantavirus pulmonary syndrome.

The disease gained attention last year after late actor Gene Hackman ’s wife, Betsy Arakawa, died from a hantavirus infection in New Mexico.

The World Health Organization said in a statement Sunday that detailed investigations of the cruise ship outbreak are ongoing, including further laboratory testing and epidemiological investigations. The virus is also being sequenced.

The virus is spread by rodents and more rarely, people
Hantavirus is mainly spread by contact with rodents or their urine, saliva or droppings, particularly when the material is disturbed and becomes airborne, posing a risk of inhalation. People are typically exposed to hantavirus around their homes, cabins or sheds, especially when cleaning out enclosed spaces with little ventilation or exploring areas where there are mouse droppings.

The WHO says that while it rarely happens, hantaviruses can also spread directly between people.

The U.S. Centers for Disease Control and Prevention began tracking the virus after a 1993 outbreak in the Four Corners region — the area where Arizona, Colorado, New Mexico and Utah meet.

It was an astute physician with the Indian Health Service who first noticed a pattern of deaths among young patients, said Michelle Harkins, a pulmonologist with the University of New Mexico Health Sciences Center who for years has been studying the disease and helping patients.

Most U.S. cases are in Western states. New Mexico and Arizona are hot spots, Harkins said, likely because the odds are greater for mouse-human encounters in rural areas.

The illness starts with flu-like symptoms
An infection can rapidly progress and become life-threatening. Experts say it can start with symptoms that include a fever, chills, muscle aches and maybe a headache.

“Early in the illness, you really may not be able to tell the difference between hantavirus and having the flu,” said Dr. Sonja Bartolome of UT Southwestern Medical Center in Dallas.

Symptoms of hantavirus pulmonary syndrome usually show between one to eight weeks after contact with an infected rodent. As the infection progresses, patients might experience tightness in the chest, as the lungs fill with fluid.

The other syndrome caused by hantavirus — hemorrhagic fever with renal syndrome — usually develops within a week or two after exposure.

Death rates vary by which hantavirus causes the illness. Hantavirus pulmonary syndrome is fatal in about 35% of people infected, while the death rate for hemorrhagic fever with renal syndrome varies from 1% to 15% of patients, according to the CDC.

A lot of unknowns about the illness and treatment
There is no specific treatment or cure, but early medical attention can increase the chance of survival.

Despite years of research, Harkins said many questions have yet to be answered, including why it can be mild for some people and very severe for others and how antibodies are developed. She and other researchers have been following patients over long periods of time in hopes of finding a treatment.

There are “a lot of mysteries,” she said, noting that what researchers do know is that rodent exposure is key.

The best way to avoid the germ is to minimize contact with rodents and their droppings. Use protective gloves and a bleach solution for cleaning up rodent droppings. Public health experts caution against sweeping or vacuuming which can cause virus particles to get into the air.