It has been a vexing question since the day Nadia was spotted dry coughing and wheezing in her enclosure at the Bronx Zoo.

How in the world did the 4-year-old Malayan tiger contract covid-19, the disease that has infected about 1.4 million humans?

Nadia is terribly unlucky — the first confirmed case of a wild or domesticated cat to come down with the disease. Her unlikely infection raised questions about whether humans can pass the coronavirus to their house pets. Zookeepers think Nadia got it from an infected human who was asymptomatic. In an interview Tuesday, Paul Calle, the zoo’s chief veterinarian, said they have no idea which human on the staff might have done it. “No one who has worked with these cats has been tested and is positive,” Calle said.

As experts scratched their heads in New York, a study released this week by the University of California at Davis explored a broader problem related to human contact with wildlife. Manipulation of the environment by humankind, the study said, has made deadly pandemics inevitable.

Hunting and domesticating wildlife, and also invading their habitats, allows the viruses animals carry to spill over, and — as Nadia’s infection suggests — vice versa. Gaming and farming brings people in direct contact with disease, while the destruction of wild habitats through urban and agricultural development sends wild mammals scurrying to areas where humans live, according to the study published in Royal Society Proceedings B.

“Deforestation, open crop fields — what we’re doing is causing wildlife to adapt to that,” said Christine Kreuder Johnson, a professor of epidemiology at UC-Davis and the study’s lead author.

Typically wildlife populations decline because of human activity, but a few of them actually grow. Out of thousands of mammals studied, the report found 58 species that increased their populations when humans altered their environments. “It’s mostly rodents,” Johnson said. “Some species of bats.”

It was a key finding when it comes to the coronavirus.

Scientists still don’t know what animal the novel coronavirus occupied before it made the jump into its first human victim in Wuhan, China. The “reservoir” population, where the virus is endemic, is almost certainly some kind of bat. Scientists have identified more than 200 coronaviruses in more than 1,000 bat species. Wet markets are where a wide variety of live animals are sold and consumed, and some in Wuhan consider bats a delicacy.

Genetic analyses suggest the coronavirus, officially SARS-CoV-2, is most genetically similar to a germ found in an endangered, scaly anteater called a pangolin. Some scientists have theorized pangolins traded on the illegal wildlife market were an intermediate host that allowed the virus to infect humans.

“Until we find a virus in a host that pretty unambiguously looks like a recent common ancestor of SARS-CoV-2, we may never know for sure,” said Tom Friedrich, a virologist at the University of Wisconsin. “There may even be more than one.”

The animal world is rife with viruses — at least 320,000 kinds are known to infect mammals alone. Through habitat fragmentation, pollution and climate change, humans have created the conditions for those viruses to flourish.

In a study published Wednesday in the journal Science, a team of researchers from China reported that domestic cats can become infected with SARS-CoV-2 and can even contract the virus through the air. The virus settles in cats’ nasal passages, trachea and lungs. “Surveillance … in cats should be considered as an adjunct to elimination of covid-19 in humans,” the researchers wrote. They also studied ferrets, dogs, pigs, chickens and ducks. Of those, only ferrets were susceptible to the disease.

It’s still rare, and a bit weird, that the coronavirus spilled from an animal to a human and then somehow back to a third animal.

On March 27, Nadia was seen coughing in her exhibit for the first time. By then the zoo had been closed 11 days. The next week her sister, Azul, two Amur tigers, and three African lions in a different facility looked sick.

In a city that has become the epicenter of the outbreak in America, staff immediately suspected the coronavirus. But they had to prove it. Nadia was tranquilized with a low-weight dart Thursday at her exhibit to allow a team of about six veterinarians and staff to gather around her.

They conducted an ultrasound, took several nose swabs and extracted blood for tests. “We wanted to rule out all the obvious causes,” Calle said. “With what’s going on in New York City with covid, we wanted that examination as well.”

Because so many sick humans are waiting to be tested amid an equipment shortage in New York, Calle felt obligated to explain that Nadia’s test was specifically designed for felines. “There’s no competition for tests,” he said.

Nadia’s samples were sent to two veterinarian schools, at Cornell University and the University of Illinois, and also to the Department of Agriculture’s National Veterinary Services Laboratories, which confirmed SARS-CoV-2, the virus that causes covid-19 in humans.

“This is the first instance of a tiger being infected with covid-19,” the lab said in a statement. The zoo’s other big cats weren’t tested because tranquilizing animals involves risk. Based on Nadia’s test, Calle said it’s assumed they, too, have the coronavirus.

In addition to tigers in public zoos, humans operate private zoos and wildlife sanctuaries with large cats, such as the freewheeling Greater Wynnewood Exotic Animal Park of Oklahoma featured in the popular Netflix show, “Tiger King.” There have been no reports of feline infection at private zoos and sanctuaries.

The cats infected at the Bronx Zoo are expected to recover. But a question remains: If a zookeeper or some other employee can infect a tiger, can people infect their house cats?

“I’m very suspicious this virus can be transmitted to cats from infected people,” Johnson said. “It’s a concern. It’s not a concern from a human risk perspective. … We need to be careful not to transmit disease to animals.”

To jump between species, the virus has to find a way to attach to the cell of a new host. It does that by binding to the receptors in cells. Receptors the coronavirus uses to attach to human cells are similar to the receptors it uses to attach to cat cells, Johnson said.

Nadia’s infection was concerning enough for the Association of Zoos and Aquariums and the American Association of Zoo Veterinarians to issue joint guidance for zoos to consider giving more personal protection equipment to animal care staffers who work with cats, “including coveralls, surgical masks, eye protection and gloves.” It also asked zoos to consider limiting access to enclosures and “immediately report anything that is unusual in the health of any of the animals.”

News of Nadia’s infection just happened to coincide with the release of the study. Johnson and her co-authors, including Tierra Smiley Evans and Pranav S. Pandit at UC-Davis, set out to advance the understanding of how people can reduce their risk of disease emergence five years ago.

Previous research didn’t ask an important question. “People show up with zoonotic diseases, and they’re not asked, ‘How did you get this?’ There was a concern about how little is reported out in the literature,” Johnson said. “This is an unprecedented situation we’re in, and I think we have to take unprecedented measures to head this off.”

The researchers concluded that humans need to find a better way to coexist with nature and reconsider the explosion of development that began in the 1940s. The coronavirus outbreak is an indication of the risks people are taking, the study said.

“It could be worse,” Johnson said. “Nature is going to determine how long we exist here. We need to understand the implications of this for our own survival.”

Staff writer Sarah Kaplan contributed to this report