China is facing the biggest and deadliest outbreak of H7N9 bird flu in human history.

The virus causes pneumonia and death in most of its victims, which is why it tops the list of global flu pandemic threats.

According to the latest assessment from the World Health Organization, China’s had 555 lab-confirmed human cases of the H7N9 bird flu virus since last October — the most of any flu season since the virus was first reported in humans in 2013.

Forty percent of those confirmed to have the virus have died, including at least 179 people in China this year alone. That high mortality rate is part of the reason the Centers for Disease Control and Prevention considers H7N9 the “most concerning” of the flu viruses it tracks, Dr. Tim Uyeki, a medical epidemiologist at CDC, told me.

Right now, the bird flu outbreak is centered around poultry markets in China, and most H7N9 virus infections have been identified in older adults who visited a live poultry market. There's no evidence yet of ongoing human-to-human transmission.

But Uyeki said that H7N9 virus could spread among poultry to nearby countries, and potentially infect humans from there. (Since 2013, some human cases of H7N9 virus infection acquired in China have been identified outside of China in Taiwan, Malaysia, and Canada.)

For now, the risk of H7N9 reaching the US is low. Still, the chances of the US being hit with some kind of pandemic in the next four years is high. (The US is already fighting Zika within its borders, where there’s been a 20-fold increase in Zika-related birth defects since the arrival of the virus here.) Based on what we’ve seen from President Donald Trump so far, the US seems poised to botch an outbreak response.

“America has long been unprepared for a dangerous pandemic,” Ron Klain, the former Ebola czar under President Barack Obama, told Vox, “but the risks are especially high under President Trump.”

Let’s walk through the four reasons why.

1) Trump hasn’t named a CDC director and could cut 12 percent of its budget

Whenever there’s a serious infectious disease outbreak, inside or even outside the US, the CDC leads the response. The scientists and researchers there are tasked with identifying outbreaks, and creating plans to stop them from spreading.

“Should a pandemic or any other public health crisis occur, we would want a team of experienced professionals in place that already knows the lay of the land, and has the administration’s trust,” said Wendy Parmet, a professor of health policy at Northeastern University. “The sooner that team forms, the better.”

Since Tom Frieden, former CDC head under Obama, resigned in January, Trump has not nominated anyone for the post. (Anne Schuchat is serving as acting director for now.)

In addition, Trump hasn’t nominated anyone for the Assistant Secretary for Preparedness and Response at HHS posting, or to head USAID — two other key agencies during a pandemic.

“Given Trump's general skepticism toward science and expertise, he is much less likely to accept recommendations from holdovers in these key positions — meaning the government might be paralyzed in the event of a crisis,” Klain said.

The rudderlessness of US public health right now is part of a much broader trend, said global health researcher Jeremy Youde. “The World Health Organization is in the midst of its search for a new director-general, and the Global Fund [the international global health financing organization] is searching for an executive director, so this is a seriously unsettled moment for any sort of outbreak to occur.”

The CDC is also poised to face some serious funding challenges: The administration’s promise to repeal the Affordable Care Act would gut a key public health fund that accounts for 12 percent of the CDC’s budget. This is part of Trump’s proposed cuts to all non-defense programs. Without appointees in epidemic prevention positions, there's no one inside the government to stand up for the programs that would help stamp out an outbreak when it sparks.

2) “America first” doesn’t work during pandemics

Trump has shown signs that he favors an isolationist approach to international relations. His questioning of NATO and retreat from the Transpacific Partnership signal “America first” is his priority — above working through the West’s post-war multilateral diplomatic architecture.

Countries can’t isolate themselves from the flow of disease-causing viruses and bacteria across borders.

Fighting and preventing pandemics requires cross-border collaboration and cooperation. Countries need to share information transparently about outbreaks within their borders. They need to agree on plans for preventing and fighting those outbreaks.

Higher-income nations like the US also have to lend resources — from researchers to funding and lab training — to their lower-income counterparts to help keep local outbreaks from spreading globally. So when the next Ebola-level threat strikes, Trump should theoretically put aside his isolationist, “America first” politics and work with other countries for the benefit of global public health.

It is possible he will. There are many examples throughout history where public health issues were put before politics to save lives, former CDC Director Tom Frieden told Vox. At the height of the Cold War, the US and the Soviet Union worked together to eradicate smallpox. During El Salvador’s civil war in the 1980s, officials created days of tranquility to vaccinate children and bring up the country’s low immunization rates. “We often find health can be a bridge,” Frieden said.

3) Trump has advocated for closing borders to countries dealing with outbreaks. That’s dangerous.

How might President Trump react to a disease threat from abroad? His tweets during the Ebola outbreak in 2014 offer some clues.

Trump called for sealing up borders and keeping health workers who had helped the Ebola effort “out of here.”

The U.S. cannot allow EBOLA infected people back. People that go to far away places to help out are great-but must suffer the consequences! — Donald J. Trump (@realDonaldTrump) August 2, 2014

“His tweets showed panic, bad judgment, and a disregard for science,” Klain said, “not a great mindset for a president facing an epidemic.”

Unlike Trump’s suggested approach, health officials discourage travel bans or other trade restrictions on countries harboring outbreaks for good reason: Such measures punish economies for circumstances that are often outside of their control, they haven’t been shown to stop the spread of pathogens

They also discourage countries from honestly reporting outbreaks within their borders.

“When there are any sanctions or negative consequences, countries will hide disease outbreaks,” public health scientist Mike Osterholm said. “Early notification of a problem should be the ultimate get-out-of-jail-free card.”

Isolationist measures would also be unhelpful for another reason: Overwhelmingly, tropical diseases are helped along because of poverty, climate change, and urbanization, Baylor College of medicine global health researcher Peter Hotez said, and shutting down borders also won’t undo those conditions.

4) The fallout from Trump’s travel ban could hamper research collaboration that could save lives

Developing vaccines or researching deadly diseases requires international collaboration, particularly in the context of an outbreak.

Consider the first successful Ebola vaccine, which, if approved, could help prevent another Ebola outbreak from ever happening: The vaccine candidate was first developed by Canadians, and during the 2013 to 2016 Ebola epidemic, West African, European, and US governments and universities worked together with the WHO and other international partners to design the clinical trials and recruit patients to find out whether the vaccine worked.

With the current H7N9 outbreak in China, we’re already seeing that the virus may be resistant to treatment. And according to Stat’s Helen Branswell, it evolved in a way that “undermines the usefulness of a 12 million-dose emergency stockpile of H7N9 vaccine made for the United States several years ago.”

In other words, the H7N9 vaccines we have for the deadly flu virus are unlikely to work, and countries are going to need to work together to develop new ones. Immigration bans shouldn’t get in the way of that life-saving work.

That’s the hope for public health. But it’s also true that in a globalizing world, an approach focused on closing borders and building walls is unlikely to be effective.

We’ll eventually learn whether Trump will put public health above politics — and hopefully the White House’s pandemic test doesn’t roll around too soon. Trump’s Ebola tweets and perpetuation of myths about vaccines certainly aren’t very encouraging. But perhaps the gravity of the Oval Office will change the stakes.