IN APRIL, THE BALTIMORE CONVENTION CENTER was scheduled to host a national cheerleading competition and a bridal expo. An international car show was on the books at the Javits Center in New York City. Instead, these cavernous facilities and others like them—built to accommodate large crowds but easily segmented into smaller spaces—have morphed into makeshift field hospitals, a place to handle the surge of patients needing care.

In other cities, other measures: In London, it’s the ExCel convention center, now a 500-bed facility. Many cities and states have begun eyeing hotel rooms and dormitories. In early March, the Vanderbilt University Medical Center in Nashville installed temporary floors and ceilings inside a parking garage, creating a 50-bed makeshift space. It allows less seriously ill patients to be evaluated and treated without entering the hospital itself, after which they can be discharged home or transferred to the emergency department if their condition warrants it.

There is an art and a science to creating such spaces, and health crises like this epidemic pull together expertise from all quarters, says architect Eric Pitts. For more than a decade, Pitts has worked as a logistician with Doctors Without Borders to build field hospitals during disease outbreaks in Sierra Leone, South Sudan and the Congo.

Pitts says the first step to building a pop-up hospital is to pick a site that people can reach. “It’s incredibly important to have proximity to the community that’s being served and to other health care facilities,” says Pitts. Ideally, the site already has access to water, protection from rain and extreme temperatures, and is equipped to get rid of waste that could carry the virus. “Making sure you have a working sanitation system can make or break a facility,” he says.

The facility should also be ready to treat a long-lasting outbreak. “We don’t want to put ourselves in a situation where the hospital can’t expand,” he says. He notes that the Javits Center and Baltimore Convention Center pop-ups both only occupy part of the available square footage, and the space left over is highly configurable.

After settling on a site, the right design inside is critical during an infectious disease outbreak. At the best of times, and even in fully outfitted hospitals, patient-to-patient transmission is a deadly risk. “In January, we saw a lot of issues in Wuhan with so-called nosocomial spread,” or infection that happens at a hospital, says Ian Jones, the vice chair of Emergency Medicine at Vanderbilt who worked on the parking garage conversion. Emergency departments were implicated in a number of those infections, according to early studies.

Photos from early days of the outbreak hinted at the reasons—long lines to get care and crowded waiting rooms. In fact, an analysis published in JAMA in early February—the same week China opened a COVID-19-dedicated hospital it had built in 10 days—reported that more than 40% of the study’s 138 patients likely acquired the infection within the hospital itself.

Understanding that can help make an impromptu hospital even safer than its brick-and-mortar counterparts, if it can be designed in a way that reduces disease spread. “You have to think about patient flow,” says Pitts. A good design will anticipate how patients come in, where they go and how they interact with providers and other patients.

The Vanderbilt site was laid out, says Jones, with safeguards to prevent that from happening at any point from entry to exit. “You do not want to have a large number of patients who potentially have this virus commingling with the uninfected,” adds Jones. Discussions about materials are also important. While the virus can live for up to a week on stainless steel, according to one recent study, metal is easily disinfected, and surfaces and supplies must be easy to clean on a regular basis.

These strategies have been applied in pop-up medical spaces around the world. But Pitts notes that even the smartest physical structure won’t guarantee success. “We’re more limited by human resources,” he says, “like filling it with qualified doctors and nurses, and recruiting the maintenance and cleaning staff.”

Jones in Nashville notes that with the situation changing daily, so can the needs required of a space. Building their assessment center took only days to finish, so it’s still a work in progress.

“We meet twice a day to discuss the needs, and trends, and what’s needed where, and this has been going on since the early part of March,” he says. From inside the hospital, he says the outbreak could develop into a serious problem with capacity and equipment required to deliver care. The final shape of the response—and its buildings—will depend on the local course of an unpredictable epidemic.