Justin Trudeau’s announcement on Monday that he would resign was the last card that Canada’s deeply unpopular prime minister, who had set his party on course to lose a national election, had left to play.

The political levers he has pulled will give Mr. Trudeau’s Liberal Party a chance to reinvent itself without him. But they will also leave Canada weakened as it braces for President-elect Donald J. Trump, who has threatened the country with tariffs that could cripple its economy.

It appears to be a gamble that Mr. Trudeau is willing to take.

To allow his party’s thousands of members to choose his successor, a lengthy process that will involve campaigning, Mr. Trudeau suspended Parliament until March 24. A general election is expected to follow.

Holding a party leadership election before a general one is par for the course in countries with parliamentary systems like Canada’s. Suspending Parliament to hold such an election is far less common. By doing so, Mr. Trudeau wards off the likely collapse of his minority government and gives the Liberals time to choose a leader unburdened by his dismal poll numbers.

But it means that in two weeks, when Mr. Trump returns to the Oval Office, Mr. Trudeau will be leading Canada as a lame duck, weakening the country’s hand in crucial negotiations with its closest ally.

“The prime minister stepping down means it will be hard for him to carry any meaningful mandate in negotiating with the U.S., and it doesn’t signal any unity within Canada,” said Xavier Delgado, a senior program associate at the Canada Institute of the Wilson Center, a Washington-based foreign policy research institute. “It’s not a great time for Canada to be in this situation.”

Mr. Trudeau’s opponents wanted a quick general election, which would let a new government with a fresh mandate — presumably led by Pierre Poilievre, whose Conservative Party has a commanding lead in polls — spearhead Canada’s response to Mr. Trump as soon as possible.

Mr. Trump has threatened to slap punishing tariffs on Canadian goods that could send the country’s economy into a recession and upend the North American trade pact established over the past few decades. (It would also be injurious to the U.S. economy; the two nations are each other’s biggest trading partners.)

The president-elect has persistently suggested that Canada should become part of the United States, calling it the “51st state.” He repeated his menacing joke on social media on Monday, after Mr. Trudeau’s resignation announcement: “Many people in Canada LOVE being the 51st State,” Mr. Trump said in a post on Truth Social, promising no tariffs if Canada “merged with the U.S.”

Party first

When Mr. Trudeau became the Liberals’ leader, the party was in shambles. In the 2011 national elections, it finished third for the first time in its history. Mr. Trudeau, who assumed its leadership in 2013, is widely credited with raising it from the dead and leading it back into government two years later.

“The Liberal Party, such as it is, has been the Justin Trudeau party for more than a decade,” said Shachi Kurl, president of the Angus Reid Institute, a research center. That has made it hard for the party to let go of him and for Mr. Trudeau to relinquish control, Ms. Kurl said.

But finally, on Monday morning, after weeks of pressure from within the party to resign, Mr. Trudeau acknowledged that his time was up.

“I truly feel that removing the contention around my own continued leadership is an opportunity to bring the temperature down,” he told reporters gathered in the freezing cold outside his Ottawa residence.

“It’s become obvious to me with the internal battles that I cannot be the one to carry the Liberal standard into the next election,” he added. Until the Liberals choose their new leader, Mr. Trudeau said, he will stay on in that role and as prime minister.

The party’s internal election process, which will last several weeks, will allow a handful of hopefuls to reintroduce themselves to the public, no longer as Trudeau associates but as individuals vying for the party’s and the country’s leadership.

“I think the Liberals are now clinging to the idea that there is no longer a path forward for him, but there is absolutely a path forward for someone else,” Ms. Kurl said.

Still, with the Conservatives leading the Liberals by 25 percentage points in recent surveys, the path that Mr. Trudeau bequeaths his successor is likely to be treacherous.

“Sixty or 90 days are not a long time to reinvent a party after 10 years in power,” Ms. Kurl said. “How many more rabbits are in the hat? How many more pivots are there?”

Brief relief

For many Canadians, Mr. Trudeau’s departure was a necessary condition if they were going to consider voting for the Liberals.

David Coletto, who leads Abacus Data, a polling company, said early research on Monday suggested that Canadians felt relief at the news of Mr. Trudeau’s resignation, and that his departure had the potential to shift attention away from his unpopularity.

“People are saying they feel relieved and optimistic about the prime minister stepping down,’’ he said. “That’s a signal to me that there’s potentially an opportunity for the Liberals to rebuild the relationship with Canadians.” But it is far from certain that this will happen, he cautioned.

While Mr. Trudeau’s departure can only improve the Liberals’ situation, analysts said, the country is unlikely to benefit from being practically leaderless as Mr. Trump takes office.

As the new president begins to push through his agenda — which has Canada in its cross hairs, with Mr. Trump having complained about border security, Canadian military spending and a trade imbalance — Canada will be trying to sort out who’s in charge.

“Canada would be strongest in dealing with the United States if it could unify around the message for its leader — and that would apply to any country,” said Mr. Delgado of the Wilson Center’s Canada Institute.

Others were less concerned, suggesting that Canada’s dealings with the Trump administration would be a long game.

Gerald Butts, a former top adviser to Mr. Trudeau who is now vice president at Eurasia Group, a consulting firm, said no leader would be able to cut a deal with Mr. Trump on Day 1.

“Nothing irreparably bad will happen in the next three months,” Mr. Butts said. “We’re going to have Trump for four years; the next three months are not going to be the whole story.”

After 12 years leading the Liberal Party of Canada, Prime Minister Justin Trudeau on Monday said he was stepping down. His resignation sets off a succession battle in which some of the party’s most prominent faces will vie to replace him.

In Canada’s political system, the leader of the Liberal Party, as the head of the largest party in the House of Commons, is also prime minister; when Mr. Trudeau’s replacement as party leader is chosen, they will assume the position as well.

That politician will take the party to general elections to face the Conservatives and their leader, Pierre Poilievre, who are dominating public opinion polls. Elections must be held no later than October, but the government, under its new prime minister, is expected to fall long before then through a vote in the House of Commons.

Mr. Trudeau said the Liberal Party would choose his successor through a vote by its members, a relatively small portion of Canada’s population. While no one has yet announced their candidacy to succeed Mr. Trudeau, here are some possible contenders.

Chrystia Freeland

The resignation last month of Chrystia Freeland, the former deputy prime minister and finance minister, incited widespread speculation that she would start her own bid to run the Liberal Party.

Ms. Freeland had a successful career in Canada as a senior editor at The Globe and Mail in Toronto and as an international journalist working as a correspondent and newsroom leader for the Financial Times and Reuters. Ms. Freeland, who was born and raised in Alberta, returned to Canada to join Mr. Trudeau’s Liberals in 2013. She is married to a reporter on the Culture desk of The New York Times.

After the Liberals subsequently came to power, she played important roles in resolving several major issues for Mr. Trudeau, in particular negotiating the successor to the North American Free Trade Agreement during the first Trump administration.

On Monday, Mr. Trudeau had kind words for Ms. Freeland, although he declined to share what transpired between them. People familiar with the events surrounding her departure said that Mr. Trudeau had attempted to fire her over a video call and offered her a minister-without-portfolio position, which she declined.

Dominic LeBlanc

Dominic LeBlanc, who became finance minister when Ms. Freeland resigned, is Mr. Trudeau’s longtime friend, and has known him since Mr. LeBlanc used to babysit Mr. Trudeau and his brothers in Ottawa. Both men were pallbearers in the funerals of each other’s fathers. Mr. Trudeau’s father, Pierre Elliott Trudeau, was once prime minister, and Mr. LeBlanc’s father, Romeo LeBlanc, had served as the elder Mr. Trudeau’s press secretary. He was later Canada’s governor general, the country’s official head of state as then-Queen Elizabeth’s representative.

Mr. LeBlanc was among the Canadian contingent who flew to Florida to meet with President-elect Donald J. Trump at Mar-a-Lago, his private club and residence, after he threatened to impose a 25 percent tariff on Canadian goods in response to border security issues. Mr. LeBlanc was first elected as a member of parliament in 2000, and has led various portfolios in Mr. Trudeau’s administration, including most recently as the public safety minister.

Mr. LeBlanc, who is from a predominately French-speaking part of New Brunswick, considered running for Liberal leadership in 2012 but changed his mind when Mr. Trudeau told him his intentions to do so.

Mélanie Joly

Mélanie Joly has been Canada’s top diplomat since 2021. Mr. Trudeau encouraged her to take the next step in her political career by accepting the foreign affairs role despite her own hesitations: Ms. Joly was concerned about how the job might affect her fertility treatments.

Ms. Joly has brought her own brand of pragmatic diplomacy to the role, reinforcing the importance of Canada working with leaders who have opposing views on foreign policy.

“There’s that movement — which I am profoundly against — which is if you don’t engage with countries, you’re sending a message that you’re strong,” she told The Times. “I think that to be strong is to be able to have the tough conversations.”

Ms. Joly has led Canada’s Indo-Pacific Strategy, but has also confronted challenges. She expelled six Indian diplomats from Canada, and contended with Chinese and Indian foreign interference allegations.

Mark Carney

Mr. Trudeau’s team had privately been pursuing Mark Carney, the former governor of the Bank of Canada who also led the Bank of England, to take over for Ms. Freeland as finance minister. Mr. Carney and Ms. Freeland are longtime friends, and he is the godfather of one of her children.

Those efforts to pursue Mr. Carney unraveled after Ms. Freeland’s public rift with Mr. Trudeau and her resignation from cabinet. Since then, Canadian news outlets reported, Mr. Carney has been calling Liberal members of parliament to ask for their support and advice if he were to make a leadership run.

During his time as governor of the Bank of England, Mr. Carney earned a reputation for sermonic talks that veered into the political, including his views on the financial dangers of climate change. In his recent book, “Values,” Mr. Carney offered a withering critique of capitalism and a proposition that markets ought to serve citizens.

Reports of a surge in cases of a respiratory virus in China have evoked dark echoes of the start of the Covid-19 pandemic almost exactly five years ago.

But despite the surface similarities, this situation is very different, and far less worrisome, medical experts say.

The Chinese cases are reported to be infections with human metapneumovirus, known to doctors as HMPV. Here is what we know so far:

What is HMPV?

It is one of several pathogens that circulate across the world each year, causing respiratory illnesses. HMPV is common — so common that most people will be infected while they are still children and may experience several infections in their lifetimes. In countries with months of cold weather HMPV can have an annual season, much like the flu, while in places closer to the Equator it circulates at lower levels all year long.

HMPV is similar to a virus that is better known in the United States — respiratory syncytial virus, or R.S.V. It causes symptoms much like those associated with flu and Covid, including cough, fever, nasal congestion and wheezing.

Most HMPV infections are mild, resembling bouts of the common cold. But severe cases can result in bronchitis or pneumonia, particularly among infants, older adults and immunocompromised people. Patients with pre-existing lung conditions, such as asthma, chronic obstructive pulmonary disease or emphysema, are at higher risk of severe outcomes.

In higher-income countries, the virus is rarely fatal; in lower-income countries, with weak health systems and poor surveillance, deaths are more common.

How long has this virus been around?

The virus was identified in 2001, but researchers say it has circulated in humans for at least 60 years. Though it is not new, it doesn’t have the name recognition of influenza, Covid or even R.S.V., said Dr. Leigh Howard, an associate professor of pediatric infectious disease at Vanderbilt University Medical Center.

One reason is that it is rarely discussed by name, except when people are hospitalized with a confirmed case of it.

“The clinical features are really difficult to distinguish from other viral illnesses, and we don’t routinely test for HMPV the way we do for Covid, flu or R.S.V.,” Dr. Howard said. “So most infections go unrecognized and are chalked up to whatever respiratory thing is going around.”

How does a person get infected with HMPV?

The virus spreads primarily through droplets or aerosols from coughing or sneezing, through direct contact with an infected individual or through exposure to contaminated surfaces — basically the same ways people get colds, flu and Covid.

Is there a vaccine? Or a treatment?

There is no vaccine against HMPV. But there is a vaccine for R.S.V., and research is underway to find a vaccination that could protect against both viruses with one shot, since they are similar. There is no antiviral treatment specifically for HMPV; treatment focuses on management of symptoms.

What is China saying about it?

The Chinese authorities have acknowledged that HMPV cases are increasing, but have emphasized that the virus is a known entity and is not a major concern. The coronavirus that causes Covid-19 was a new pathogen, so people’s immune systems had not built up defenses against it.

At a news conference held by China’s Center for Disease Control and Prevention on Dec. 27, Kan Biao, the director of the center’s Institute for Infectious Diseases, said that HMPV cases were rising among children 14 years and younger. The increase was especially notable in northern China, he said. Influenza cases have also increased, he said.

Cases could spike during the Lunar New Year holiday, at the end of January, when many people travel and gather in large groups, he said.

But overall, Mr. Kan said, “judging from the current situation, the scale and intensity of the spread of respiratory infectious diseases this year will be lower than last year’s.”

Official Chinese data shows that HMPV cases have been rising since mid-December, in both outpatient and emergency cases, according to Xinhua, the state news agency. Some parents and social media users were unfamiliar with the virus and were seeking advice online, the outlet said; it urged calm and ordinary precautions such as washing one’s hands frequently and avoiding crowded places.

In a routine media briefing on Friday, a spokesperson for the Foreign Ministry reiterated that cases of influenza and other respiratory viruses routinely increase at this time of year but that they “appear to be less severe and spread on a smaller scale compared with the previous year.”

Chinese officials said last week that it would set up a monitoring system for pneumonia of unknown origin. It will include procedures for laboratories to report cases and for disease control and prevention agencies to verify and handle them, the state broadcaster CCTV reported.

What has the public response in China been?

Online, amid comments from people saying they had never heard of HMPV and expressing concern that it was a new pathogen, state media outlets have sought to reassure people, warning them against blindly taking antiviral medications.

Some users have cracked jokes, saying that they could finally use up the masks they had stockpiled during the coronavirus pandemic. Many commenters have discussed a general uptick in illness, not just HMPV: “Why does the flu hurt so much” was trending on Weibo, a social media platform, on Monday.

What does the World Health Organization say?

The W.H.O. has not expressed concern. Dr. Margaret Harris, a spokeswoman for the organization, cited weekly reports from the Chinese authorities that showed a predictable rise in cases.

“As expected for this time of year, the Northern Hemisphere winter, there is a month-over-month increase of acute respiratory infections, including seasonal influenza, R.S.V. and human metapneumovirus,” she said by email.

Should I worry?

The reports coming from China are evocative of those from the first, confusing days of the Covid pandemic, and the W.H.O. is still urging China to share more information about the origin of that outbreak, five years on.

But the current situation is different in key respects. Covid was a virus that spilled over into humans from animals and was previously unknown. HMPV is well studied, and there is widespread capacity to test for it. There is broad population-level immunity to this virus globally; there was none, for Covid. A severe HMPV season can strain hospital capacity — particularly pediatric wards — but does not overwhelm medical centers.

“However, it is also vital for China to share its data on this outbreak in a timely manner,” said Dr. Sanjaya Senanayake, a specialist in infectious diseases and associate professor of medicine at the Australian National University. “This includes epidemiologic data about who is getting infected. Also, we will need genomic data confirming that HMPV is the culprit, and that there aren’t any significant mutations of concern.”

Vivian Wang contributed reporting from Beijing.