Cruises are so closely associated with illness that the highly contagious norovirus is commonly called the “cruise ship virus.”

But a ship headed for Spain’s Canary Islands has attracted global attention due to a rare outbreak of hantavirus that’s left three dead. While alarming, health officials and infectious disease experts say the risk to the general public right now is low because hantavirus is less contagious than other respiratory diseases like the coronavirus responsible for the Covid-19 pandemic.

“This is not Covid, this is not influenza. It spreads very, very differently,” Maria Van Kerkhove, director of epidemic and pandemic preparedness and prevention at the World Health Organization, said at a press conference on Thursday.

During the briefing, WHO’s director-general Tedros Adhanom Ghebreyesus confirmed eight hantavirus cases among passengers of the MV Hondius luxury cruise ship, including the three who died. Typically transmitted by rodents, hantavirus can cause severe disease in humans. People usually get sick by inhaling air that’s contaminated with droppings, urine, or saliva from infected rodents. But the particular strain identified in the cruise ship cases, called the Andes virus, can spread between people.

Health officials in several countries are working to trace the contacts of 29 people who disembarked the ship on the remote South Atlantic island of St. Helena on April 24, about two weeks after the first hantavirus death occurred. A Swiss man who left the ship early has tested positive for the virus and is being treated, and two people in the UK are reportedly self-isolating after returning home. Six people from the US were among those who got off the ship.

“The Administration is closely monitoring the situation with U.S. travelers onboard the M/V Hondius cruise ship with confirmed hantavirus,” the Centers for Disease Control and Prevention said in a statement on Wednesday.

Yet experts say there’s no need to panic at this point.

“It doesn’t spread terribly well, so I don't have any concerns of this being the next Covid,” says Steven Bradfute, an immunologist and associate director of the Center for Global Health at the University of New Mexico. “Most of the spread in the past with this virus has been with close contacts—people sharing a bed, people sharing food, that sort of thing.”

The virus doesn’t spread easily with casual contact, and asymptomatic spread—a major driver of Covid cases during the pandemic—is also less likely. The available data on the Andes virus suggests it’s most likely to be transmitted when somebody is visibly sick, Bradfute says. Symptoms include fever, muscle aches, fatigue, and dizziness, which can progress to coughing, shortness of breath, and difficulty breathing.

“That is actually really helpful, because it makes it a lot easier to do contact tracing and to identify high-risk individuals,” he says, though he cautions that outbreaks of Andes virus are uncommon, and just because the virus has behaved one way in the past does not mean it always will. “The infections have been rare enough that we can’t say that with certainty.”

One of those outbreaks occurred from late 2018 into early 2019 in Patagonian Argentina, stemming from a birthday party attended by around 100 people. Three people were the main drivers of the outbreak, which resulted in 34 cases and 11 deaths. The authors of a study who traced the outbreak in detail found that 26 of the 34 cases became sick after close contact with someone who was infected, including people who hadn’t attended the party. Six people were likely exposed to the virus via droplets or aerosols.

Hantaviruses are found all over the world, with “Old World” strains in Africa, Asia, and Europe causing kidney disease and “New World” strains in the Americas causing respiratory disease. The Andes strain is endemic to Argentina, where the ship departed from. In the US, most hantavirus cases occur in the Southwestern states when people come into contact with rodent droppings. Its high fatality rate of around 35 percent is what makes the new outbreak concerning.

“The lungs fill with fluid because the blood vessels start leaking, and once the lungs start filling with fluid, you can't breathe because there's not efficient exchange of the air,” says Scott Weaver, a hantavirus researcher at the University of Texas Medical Branch and the Global Virus Network, an international coalition of virologists. “Without proper treatment, and even with good treatment, patients can die.”

Despite a Swiss man returning home and testing positive for the virus, public health officials there say the risk to the public is low, noting that the disease is known to spread through close contact. The patient’s wife, who traveled with him, has not yet shown any symptoms but is self-isolating as a precaution. “There is currently no danger to the Swiss public,” the country’s Federal Office of Public Health said in a May 6 statement.

Health officials are still investigating how the outbreak on the cruise ship began. Investigators with the Argentine government told the Associated Press that the Dutch couple may have been infected during a bird-watching outing in the city of Ushuaia before boarding. Argentina's ​health ministry said in a statement that it will trap and test rodents for the Andes variant in the city.

“I don't think anybody should be worried about getting on a typical cruise ship and being infected with the hantavirus,” Weaver says. “This was a unique circumstance where the ship docked in a part of the world where these viruses are present, and most of the big cruise ships don't go to this part of the world.”