On Tuesday, Feb. 18, no coronavirus cases had been reported in Iran. On Sunday, the government announced 43 cases and eight deaths. Some 152 cases (and at least three deaths) were confirmed in Italy on Sunday, up from three cases on Thursday. The number of infected people in South Korea jumped to 763 (and six deaths) in just days.

As of Monday, Covid-19 was detected in at least 29 countries. In nations with few or no reported cases so far, particularly in South America and Africa, the absence of evidence shouldn't be interpreted as evidence of absence. More likely, it reflects lack of testing.

Is the Covid-19 outbreak now a pandemic, whether or not the World Health Organization calls it that yet?

If so, what's next?

First, let's get the facts straight about what can and cannot be done.

It's now clear that the epidemic was never going to be contained. At most, its spread was slowed by the lockdown imposed in China and other countries' efforts to identify infected people and anyone they might have been in contact with.

Covid-19 seems to spread like influenza, through the air, person to person. Unlike Ebola, SARS and MERS, individuals can transmit this coronavirus before the onset of symptoms or even if they don't become ill. An infected person appears to spread the disease to an average of 2.6 people. After 10 generations of transmission, with each taking about five or six days, that one initial case has spawned more than 3,500, most with no or mild symptoms, yet probably infectious. The fact that mild cases are difficult to differentiate from colds or the flu only complicates the diagnosis.

In light of the disease's features, the quarantine of the passengers and crew members on the Diamond Princess cruise ship in Yokohama Bay in Japan looks like a cruel experiment: While confined, these people were forced to breathe recycled air for two weeks. The measure achieved little except to prove just how effective the virus is at spreading. Trying to stop influenza-like transmission is a bit like trying to stop the wind.

Vaccines are many months away, at the earliest. And based on previous experiences with SARS, MERS and pandemic influenza, there is no reason to believe — as President Donald Trump claimed — that Covid-19 will go away this spring as warmer weather arrives in the Northern Hemisphere. Transmission around the world could continue for months.

The lockdown imposed by the Chinese government in Hubei, the province worst hit by the disease, substantially reduced the number of new cases for a time. But even that has limited benefits. As China tries to return to work, public transportation resumes and citizens start moving about, there will likely be a major rebound in cases. Unless an entire population shelters in place for many months, infectious agents like influenza or this coronavirus will find people to infect.

In other words, a lockdown is mostly a delaying tactic. By distributing cases over time, it can help manage an outbreak — but only if it takes place against the backdrop of a robust health care system. Yet even the best system is too fragile, and a moderate increase in infectious cases, whether from a seasonal flu or Covid-19, can quickly overwhelm resources, in China or the United States.

As chilling as it is to imagine this scenario, what happened in Wuhan, the Chinese city at the epicenter of the outbreak, will likely play out elsewhere, too. Hospitals might have to turn away all but the people most seriously ill; their ability to handle their usual load of patients with heart attacks, critical injuries or cancers may be severely compromised.

In a world ill-prepared for a potentially life-threatening, easily transmitted disease like Covid-19, the most effective way to mitigate the pandemic's impact is to focus on supporting health care systems that already are overburdened.

This is the main reason every country's top priority should be to protect its health care workers.

The United States and other countries in the Northern Hemisphere already are in the throes of a moderately severe flu season. Their inventories of protective equipment used by doctors, nurses and emergency medical workers — N-95 respirator masks, gloves, eye protection, disposable suits — are running low. These limited supplies must go to health care workers first, rather than the public. This is partly to ensure that hospitals themselves do not become sites where the coronavirus is spread more than it is contained: If infected health care workers die in large numbers, entire societies may be shaken to the point of panic.

Governments should also conduct Covid-19 preparedness drills in local hospitals and expand hospitals' temporary capacity, for example, by setting up emergency tents in parking lots, as is already happening in some places in the United States. To minimize the strain on overstressed acute-care hospitals, supportive nursing care might have to be provided, in makeshift facilities and patients' homes, as was done during severe pandemics in the past, such as the Great Influenza of 1918-19.

The manufacturing and distribution chains for drugs and other vital products like needles and syringes must remain open, and that, given the global nature of the industry, requires international cooperation. In keeping with World Health Organization guidelines, coronavirus-stricken countries shouldn't be walled off the way that the United States and others are trying to do with China at the moment. Otherwise, as the virus spreads, we will be isolating ourselves, too, and will jeopardize our ability to obtain critical resources. Many of the active ingredients in lifesaving generic drugs — the ones that stock hospital crash carts and maintain our daily well-being — come from China and India. If that production is brought to a standstill, many people could die, not directly from Covid-19, but indirectly from a lack of access to those drugs.

Ensuring all of this means facing the hard facts of this unfolding pandemic — and that requires thorough, transparent disclosures to the public. Past experiences, with the anthrax-laced letters in 2001 and the 2014 Ebola outbreak, suggest that people react more rationally and show greater resilience to a full-blown crisis if they are prepared intellectually and emotionally for it.

And yet even those officials and experts who have candidly predicted a pandemic are not saying enough about what to expect and how to prepare. Basic information is still lacking, or isn't getting through: According to a recent survey, 65% of people in Hong Kong had enough surgical masks for a month or more — this, even though such masks will do little to prevent the spread of Covid-19.

Singapore, which is experiencing an outbreak despite a world-class medical and public health system — 89 cases as of Sunday — is the model to emulate. It is preparing its citizenry for a greater crisis still by providing it with explicit instructions and specific advisories about, for instance, attending large gatherings or sharing residential areas with people under home quarantine.

And what should each of us do, beyond staying informed and washing our hands frequently? Keep calm and rational. It might be worth stocking some reserve of critical medications, for example — but not too much, because hoarding could create shortages.

We, as individuals, can also try to plan for basic contingencies. Companies can cross-train key staff members so that one person's absence won't derail the business. Family members and friends should be watchful of one another's health and welfare, and stand prepared to care for the moderately ill if hospitals become overtaxed.

"Pandemic" isn't just a technical public health term. It also is — or should be — a rallying cry.

Michael T. Osterholm is director of the Center for Infectious Disease Research and Policy at the University of Minnesota. Mark Olshaker is a writer and documentary filmmaker. They are the authors of "Deadliest Enemy: Our War Against Killer Germs." They wrote this article for the New York Times.