It probably won’t surprise anyone to learn that the experts are divided – in this case, on the question of whether cities are good for humanity’s health.

“It gets into a whole nasty debate among public health researchers,” said Matt Boyce, a senior research associate at Georgetown University who studies urban health.

On the plus side: Cities have expansive public health systems, higher concentrations of health care providers, and large, sophisticated facilities.

On the downside: Cities cram people more closely together, in worse air, in conditions that haven’t always been – and aren’t always now – particularly sanitary.

Perhaps most relevant to the present moment, cities have always been places where infectious diseases – from the bubonic plague to SARS – spread more quickly.

“It is clear that diseases and cities have been tied at the hip as long as people have been living in cities,” said Roger Keil, a professor of urban and suburban studies at York University.

That includes the Covid-19 pandemic, which began in a megacity – Wuhan, in China’s Hubei province – and quickly swept around the globe.

Regardless of whether they’re healthy places to live, humans in the modern era are indisputably drawn to cities. More than half the world’s population currently lives in an urban environment, according to the UN. In 1800, that number was 2%, Keil said. In 2050, it will be 70%.

That may mean that there are more major outbreaks in our future – and that they may spread more rapidly than ever.

Penalties

Originally, Keil said, living in cities brought an “urban health penalty,” caused by, among other things, pollution and close, unsanitary conditions.

At the beginning of the 20th century, that changed. “In industrialized countries and those who were more developed, it was now possible to develop public health and sanitation measures that turned the urban health penalty into an advantage,” Keil said. “Now you had clean water and a sewer system and a public health facility and parks.”

It was only at the end of the century, when humans began to move in huge numbers into cities – pushing their limits outward into previously remote spaces – that urban health began to be more unsafe, frequently because of diseases as varied as E. coli and HIV/AIDS, both of which saw substantial urban outbreaks.

Then came SARS in 2003. The disease struck Toronto particularly hard. “For the first time, [SARS] brought back this idea that if you are living in cities, whether you’re in a global south or a global north, whether you’re in the east or the west…all of a sudden we discovered that we are vulnerable,” Keil said.

Within and beyond

Today, Boyce said, cities are particularly good at spreading diseases quickly and efficiently – a combination of population density, high levels of movement, and social inequality.

Covid-19 is a particularly good example of the first two.

“Dense populations are kind of a hallmark of cities, but when you’re talking about something like respiratory disease,” that dense population means a spread that’s exponentially quicker than in more isolated environments, Boyce said.

Wuhan, a megacity of 11 million people, was the perfect atmosphere for a disease that relies on relatively close contact.

It’s also telling that, before the Chinese government placed Wuhan on lockdown, an estimated 5 million people had traveled from the region.

“Diseases can get around the world quickly, and they do so primarily by spreading through cities because that’s where our big airports and seaports and train stations are,” Boyce said. “There are thousands to millions of people and animals and goods moving through these places every single day.”

That’s a recipe for spreading disease, not only within a contained population, but to other places as well.

Finally, Boyce said, the conditions that confront the poorest city-dwellers are particularly worrisome during an infectious disease outbreak. Although urban slum populations haven’t been especially hard-hit by Covid-19 yet, it’s a serious concern.

“You’re telling people…to spread out six feet from one another, and self-isolate, and wash your hands,” he said. But those instructions aren’t relevant to people living in cramped quarters with no running water.

“You don’t have access to clean water, so you can’t wash your hands. [You’re] in a one-room structure with eight other people, so you can’t spread out.”

In Mumbai’s slums

That’s precisely what concerns Anita Patil-Deshmukh, executive director of Pukar, a Mumbai-based research institution focused on the impacts of urbanization.

Mumbai has a large number of urban slums – fast-growing, densely populated neighborhoods, frequently without sanitation or running water, on the outskirts of the city.

Patil-Deshmukh said that, in these neighborhoods, the average dwelling is about 100 square feet, and may be occupied by as many as 10 people. In these circumstances, social distancing and safe hygiene are “a luxury that you and I can enjoy – not them.”

In addition, she said, Mumbai’s slum residents tend to be malnourished and already in poor health.

But while Covid-19 hasn’t gained a strong foothold in Mumbai’s slums, Patil-Deshmukh explained that its economic effects are already causing devastation.

“A large percentage [of people in Mumbai’s slum neighborhoods] are daily wage workers, or cab drivers, or cleaners in homes…or construction workers. Now, with this pandemic, all of these industries have come to a dead halt,” she said.

“The heaviest price of this pandemic and the economic turndown is going to be paid by the poorest people in India. A lot of them are slum dwellers.”

A global phenomenon

But Keil didn’t emphasize the vulnerability of urban slum residents.

“The inhabitants of these dense informal settlements are actually quite good at organizing themselves,” he said. “These people are not helpless victims. They’re not ignorant communities.”

As an example, Keil cited the West Point slum of Monrovia, Liberia. West Point, which houses a population of about 75,000 people, was placed under a government quarantine in 2014, in the midst of the West Africa Ebola outbreak.

Faced, with the prospect of Ebola’s spread in their community, Keil explained, “people had to self-organize…from the bottom up. They were self-trained and community-trained individuals” who organized contact tracing and less-invasive ways to care for sick loved ones.

“Imams and notable people in the community became heroes in fighting the Ebola outbreak, just by going from door to door,” Keil said.

Instead, for Keil, we need to redefine what we think of “urbanization.” The world is no longer divisible into countries and cities.

“When we talk about urbanization…we are now talking about urbanization as a global phenomenon that spans the globe and also reaches into areas that have never been urbanized,” he said. In particular, industry and human settlements are pushing further and further into the forest and other previously wild areas.

The result is more contact with the animal kingdom. According to Keil, it’s no accident that Covid-19, like Zika and Ebola, is a zoonotic disease – transmitted between animals and humans.

“Zoonotic diseases that had never been in the human world…are now shaping the discussion and shaping the reality of the relationship between disease and urban development,” he said.

“The bubble has burst around safety and security…and we are starting to see vulnerabilities that for the longest time were hidden.”