The saga of New York development is often told in terms of power brokers and real estate tycoons, billion-dollar construction and decades-long master plans. But one of the major forces behind much of the city’s built environment is far less conspicuous: microbes. These invisible microorganisms influence how we experience virtually every element of the city today: housing, urban design, plumbing, public space, streets, sanitation, and even our symbolic visual language.

“We can’t understand our city and how it developed without thinking about the smallest New Yorkers: the microscopic New Yorkers,” says says Sarah Henry, deputy director and chief curator of the Museum of the City of New York (MCNY). From now until April 2019, the museum is presenting “Germ City: Microbes and the Metropolis,” an exhibition co-organized the New York Academy of Medicine and Wellcome, a biomedical research charity, about how infectious disease influenced the city physically, culturally, and sociologically.

“The conversations on microbes and pathogens got very deeply embedded into bigger conversations,” Henry says, referencing political issues like race, immigration, war, and civil unrest. “You can’t untangle them.”

During the 1800s, epidemics swept the city. Diseases like cholera, measles, scarlet fever, tuberculosis, and smallpox killed thousands. The city relied on local water sources, which were frequently contaminated. After planners began to understand the connection between dirty water and recurring epidemics, they constructed the Croton Aqueduct, located north of the city, to bring fresh water into the growing metropolis from upstate. Today, the city relies on the same system.

In the late 19th century, more than 100,000 horses worked on New York City streets and produced around 3 million pounds of manure every day, which would pile up and attract flies that spread typhoid, cholera, salmonella, and dysentery, among other illnesses. The city appointed former Civil War colonel George Waring to be sanitation director and he cleaned up the streets and established a modern sewer system.

While the connection between illness and infrastructure is fairly linear in some cases—like sewage and water—it wasn’t always so explicit. Scientists and doctors often didn’t know how disease spread and placed the blame, incorrectly, on specific groups of people, particularly immigrants and the poor.

During research for the exhibition, curators often encountered “complicated ethical challenges in protecting the population at large and respecting individual freedom,” says Rebecca Jacobs, one of the curators. “Hygiene could be a form of social control and support health.”

The overcrowded living conditions in tenement housing was considered a breeding ground for disease. One area with high rates of infection was nicknamed the “Lung Block” and other areas were called “Fever Nests.”

“It’s a complicated story because these perceptions around tenements were used to stigmatize immigrants,” Henry says. A disproportionate amount of resources were spent studying epidemics and living conditions in poorer areas while the upper and middle class were understudied.

“Someone who was well off could hide their illness,” Jacobs says. “Yet the consequences of these housing reforms meant that tenements were demolished and people were displaced and unable to find proper housing.”

The city enacted a number of building codes and regulations to improve living conditions and curb the spread of disease. For example, building codes were updated to mandate air shafts and a certain amount of windows. At the time, regulating what landlords did with private property was highly controversial, but policymakers framed the issue as one of self-preservation: If property owners didn’t improve conditions in low-income areas, they would get sick themselves.

“It wasn’t charity, it wasn’t humanitarian—it was enlightened self-interest,” Henry says. “There was a lot of victim blaming in the rhetoric and assumptions on who was at fault. Nevertheless, it’s still important part of our history.”

Parks and public space were developed as a way to give New Yorkers access to fresh air and space, which were (and still are) viewed as essential to health and wellness. Even New York’s expansion northward and the development of the grid could be traced to microbes.

“Why did people want to develop upper Manhattan? Why lay a grild eight miles away [from where people lived]?” Henry says. “Dispersing density and crowding is an investment in longevity and health. It was also a great real estate deal and a move for the political parties—it’s all tangled up.”

As science improved and doctors began to learn how to treat epidemics with medicine, anti-viral drugs, and immunization, the response to microbes changed scales.

“It’s not surprising the major built environment pieces came in the 19th century,” Henry says. “It was a blunter tool and a more macro kind of response because there wasn’t a way to target the microbes on a micro level. They were looking at big systems: water, sanitation, and housing.”

However, the threat of spreading disease continued to inform urban design even though science didn’t necessarily support the claims.

The racially biased discussion around illness and disease also contributed to urban renewal practices in the 20th century. “Slum clearance” programs—which bulldozed established neighborhoods of mostly African American and Latino families—were rationalized as a way to improve public health and make way for more “healthy” New York City Housing Authority apartment towers, which, due to discriminatory housing policies, would be rented mostly to white families for decades. Works Project Administration propaganda posters from the 1930s proclaimed: “Planned Housing Fights Disease.”

There are also a number of smaller ways microbes impacted the city as a whole. For decades, people drank from New York City water fountains using an attached cup before they were banned for hygienic purposes. Spitting was banned as a result of the 1918 influenza epidemic.

Later in the 20th century, epidemics continued to impact the city’s built environment. As HIV/AIDS began to spread, the city closed bath houses. Organizations developed outreach programs—which often included PSAs posted around the city—to help spread awareness and promote safe sex. Today, the descendants of these efforts are still visible in subway and street ads and signage.

One of the most recent citywide design initiatives in response to microbes comes courtesy of graphics: the letter grades posted on restaurant windows. The program only began in 2010 but has since become a widely understood way to evaluate how clean a restaurant is. Designed by Jose Torres, then an intern at the Department of Health, the “A” “B” “C” or “Grade Pending” signs are ubiquitous today. MCNY’s exhibition includes one of these signs as well as an “Employees Must Wash Hands” sign.

“It is a reminder of the importance of design in efforts to inform the public and basically communicate in a simple and clear way issues major public health importance,” Jacobs says.

While epidemics are no longer as deadly as they were in the 1800s, they’re certainly not eradicated; this exhibition is intended to help people better understand how New York has responded to them in the past, as well as provoke thinking about them in the future.

“One should not forget the prior struggles with epidemics,” Henry says. “There will certainly be a next one.”

“Germ City: Microbes and the Metropolis” is on view at the Museum of the City of New York until April 2019. Visit mcny.org for more.