The novel coronavirus has now reached the level of a global pandemic. That does not, however, mean the spread of the virus is impossible to stop.

“The danger is that if you do not try and stop this, it can overwhelm your health system,” Michael Ryan, executive director of WHO’s Health Emergencies Programme, said in a press conference on March 11th. “There have to be very strong efforts made to suppress infection, to push the infection back.”

Experts say the best defense we have is social distancing — asking people to avoid gathering in groups and keep at a distance from others.

Even before the pandemic declaration, groups were canceling large gatherings to try to slow the spread of the virus. SXSW, the annual event held each year in Austin, Texas, was canceled one week before it was set to start due to concerns about the novel coronavirus. The Swiss government banned gatherings of 1,000 people or more, shutting down the 2020 Geneva Motor Show. Emerald City Comic Con, which was set to take place from March 12th–16th in Seattle, was postponed. In New York City, Mayor Bill de Blasio asked people to work from home if they’re able and to avoid crowded subway cars during rush hours.

Canceling events and encouraging people to stay out of crowded areas may seem like overkill in places that haven’t had a high number of identified cases of the virus, but evidence from past outbreaks shows that implementing policies that keep people at a distance can prevent one from exploding.

During the 1918 influenza pandemic, for example, fewer people died in cities that closed churches and schools early on. Cities like St. Louis, which closed movie theaters and canceled sporting events when only a few people had the flu, were able to reduce the spread of disease by around half. Philadelphia, which decided to hold a parade at the start of the outbreak, saw the number of cases skyrocket.

There still may be high numbers of cases — and even deaths — in places that put early social distancing policies in place during an epidemic. Crucially, though, those cases are spread out over a longer period of time. Epidemiologists refer to it as flattening the curve. That prevents a sudden surge in the number of sick people, which can quickly overwhelm hospitals and the health care system. A flattened curve means there aren’t as many people sick at once, so they can be cared for and discharged. The outbreak may go on for longer, but the impacts wouldn’t be as severe.

Even though earlier may be better, there’s no consensus on the best time to put social distancing policies in place. They can be socially and economically taxing, and it’s emotionally difficult for people to see everyday life disrupted around them. China locked down cities and kept people in their homes for weeks to try to beat back the outbreak, which came at a huge psychological cost — even though it probably contributed to the decline in the number of new cases they’re seeing each day.

Recommendations in place in the US range from canceling events to asking the elderly, who are at risk of having severe illness from the new virus, to avoid traveling unless it’s necessary. Some school districts shut down, and some universities moved their classes online. Many companies are asking their employees to work from home.

But in the US, applying public health policies can be especially challenging. Many people don’t have paid sick leave and can’t afford to stay home or off of crowded public transportation if they’re feeling ill. Shutting down schools might seem like an easy way to stop the spread of disease, but the high cost of child care might mean doing so would cause more harm than good. If parents can’t stay home from work, they might leave kids at community centers, like they did during the H1N1 epidemic in Seattle. New York City, for example, is reluctant to close all schools in response to the COVID-19 outbreak because they’re the only places many kids get hot meals each day.

Applying social distancing measures to the extent the system allows, though, is the best defense against the spread of the novel coronavirus, particularly without vaccines or treatments available. The goal isn’t to eliminate the disease entirely, but to keep cases from appearing all at once. During a pandemic, that can mean the difference between a severe outbreak and one that’s more manageable.