Coronavirus has shut down schools and events. Here’s why that helps, short- and long-term.

Trips are being canceled. A few schools have closed. Employees are being urged to work from home.

A fair question is: Why?

Sure, these measures would prevent some coronavirus infections in the short term. But what’s to keep the virus from roaring back if we stop restricting behavior a few months from now?

The concept is called “social distancing,” and there are three broad reasons it can help by spreading out infections over a longer period of time, public health experts say.

The reason for that last potential benefit takes a bit of explaining, and it depends on which kinds of restrictions are put in place and when, and how easily the virus is transmitted from person to person. But there is little dispute that when society is faced with a new infectious agent to which no one has immunity, it is crucial to buy extra time.

Carolyn C. Cannuscio, a social epidemiologist and associate professor at the University of Pennsylvania’s Perelman School of Medicine, urged her students in a class Wednesday to weigh some of the options.

On a screen behind her, she projected a graph with two “peaks" to illustrate what can happen in a pandemic — the term for an epidemic that is spreading globally.

One illustrated what can happen when widespread event cancellations and other restrictions are put in place: a lower overall number of cases spread out over a longer period of time.

The other showed what can happen without such measures: an onset of cases that was “fast and furious," she said.

“These early cases become the fuel for the epidemic. How can we get the curve to look more like this low and slow outbreak, rather than this fast and furious outbreak?” she asked her students, who represented a variety of fields, such as medicine, nursing, and veterinary science.

The graph was a general illustration drawn from experience with the flu. The exact shape of the curves for the new coronavirus remains to be seen, but experts have estimated that China’s widespread restrictions on travel and public gatherings prevented many thousands of cases.

Cannuscio and colleague Alison M. Buttenheim, an associate professor at Penn Nursing, reminded the students to think of the human cost of shutting down schools and other programs, urging them to consider a less disruptive approach when possible.

Delay large events that involve students from multiple schools, perhaps. Split classes into smaller groups, or put students on rotating schedules. Or, hold certain athletic events as scheduled but close them to fans, as host Amherst College did Friday for a first-round NCAA championship basketball game featuring Rowan University.

“We tend to think in a dichotomous way: Either schools are open or schools are closed,” Cannuscio said. “I want us to be thinking about a gradient.”

So far, children do not seem to be hit hard by the coronavirus. But they can transmit it to older people and others who are more vulnerable, so curtailing school activities may be a smart way to intervene. Many hospitals operate near capacity, so every infection that is prevented allows more “slack” in the health-care system, Buttenheim told the Penn students.

At a news conference earlier in the week, Harvard epidemiologist Marc Lipsitch also spoke about the best practices for social distancing. It is not always clear how long to keep such measures in place, but lifting them too soon is a recipe for trouble, he said.

“When you let up on those interventions, then transmission resumes,” said Lipsitch, a professor at the Harvard Chan School of Public Health. “The virus doesn’t know that people were social distancing last week."

For those without a public health background, it might not be obvious why spreading out the cases also can result in fewer cases in the long run.

The reason is that viruses are creatures of opportunity, said Maciej Boni, a Pennsylvania State University biologist who studies infectious diseases. They will spread as long as there are people nearby who have yet to develop immunity.

But after a virus infects a certain percentage of a population, any remaining sick people will encounter fewer and fewer susceptible people in close proximity, and the virus loses steam.

If no social distancing measures are enacted, then the virus spreads faster, potentially overshooting that percentage where it would lose steam, the Penn State biologist said. Far better to restrict public behaviors, so the virus infects no more people than necessary in order for it to run out of gas.

“You want a soft landing,” Boni said.

Ideally, the bulk of infections from a new virus can be delayed until drug companies develop a vaccine. More than a dozen research teams are at work on that task, with a few promising to start human tests by early summer. But experts have predicted that a vaccine will not be ready for widespread use for at least a year.

» READ MORE: Inside a Philadelphia lab, scientists race to test a coronavirus vaccine

In the meantime, that means treating sick patients as best we can, and preventing them from infecting others.

In the early days of the coronavirus outbreak, the United States and other countries restricted travel to and from China, where the virus originated.

But before long, travel restrictions will be irrelevant, because the risk of being infected in most places around the world will be no higher than it is in the United States, Harvard’s Lipsitch said. Now we have to look inward.

Paraphrasing from an Australian blog called “Virology Down Under,” he said:

“We’re moving now from the phase of keeping them from infecting us to the phase of keeping us from infecting each other.”