The most important factor in enabling the spread of pandemics in slums is the neglect of these marginalized populations by governing elites.

Although the Covid-19 pandemic was spread around the world by people who could afford to travel by airplanes and cruise ships, the coronavirus now threatens the socially invisible and forgotten communities of urban slums.

About a billion people live in slums — defined by the United Nations as human settlements with inadequate access to safe water and sanitation, poor housing quality, overcrowding and insecure residential status.

Infectious disease epidemics tend to spread into slums. Cases of Covid-19 infection have already been reported from Dharavi in Mumbai, India; Orangi Town in Karachi, Pakistan; and Payatas in Manila. The Ebola epidemic in West Africa between 2014 and 2016 was largely driven by the virus entering the large and densely populated urban slums of Liberia, Guinea and Sierra Leone.

During epidemics, slum residents are much more vulnerable to respiratory infections such as influenza and dengue. In a 2018 report from Delhi, for example, researchers estimated that even with widespread vaccination and social distancing (staying home, closing schools, isolating the sick), slum populations would suffer 44 percent higher influenza infection rates than those in non-slum communities.

Crowding is an important reason for the increased infection rate in slums: The population density of slums in Delhi, for instance, is 10 to 100 times higher than that of non-slum communities, and nearly 30 times that of New York City. Undernutrition of children and high prevalence of chronic medical conditions in adults make residents further susceptible to infection.

Limited access to sanitation services is another problem: In slums that lack enough clean public toilets, the coronavirus could spread through stool. And access to clean water is yet another, one which the preventive restriction of movement could actually exacerbate.

What’s more, indoor air pollution caused by cooking with biomass fuel in poorly ventilated or windowless homes can contribute to chronic respiratory conditions — increasing the risk of severe coronavirus infections for residents.

But the most important factor in enabling the spread of pandemics in slums is the neglect of these marginalized populations by governing elites. There is previous little effort to prevent the spread of diseases. Access to tests for the coronavirus, for example, is extremely limited.

Interventions need to be carefully considered. Without taking into account the specific needs of slum dwellers, measures such as social distancing are impractical. Policies need to be carried out in partnership with community organizations — such as Slum Dwellers International, a global network of community-based organizations representing slum dwellers that has mapped many settlements. Even local gangs could be partners. In the slums of Brazil, some gang groups have built hand-washing stalls at the main entrances to some slums.

And once an epidemic becomes established in slums, its magnitude is at risk of being underestimated — fueling a further spread of contagion. For instance, researchers in Delhi showed that if slums were ignored, infection rates in the city would be underestimated by 10 to 50 percent, and that the effectiveness of vaccination may be overestimated by 30 percent to 55 percent.

The underestimation of the magnitude of infections in slums may lead to unequal allocation of health care resources. Advanced life support — intensive care units and ventilators — is less likely to be available for slum residents who suffer severe cases of Covid-19.

Then there’s the economy. The negative economic effects of a coronavirus epidemic will be disproportionately felt by residents of slums. A large segment of the slum work force participates in the informal economy, which would essentially disappear under lockdowns.

In the slums of New Delhi, Mumbai, Cape Town, Manila, Karachi, Rio de Janeiro and Nairobi, Kenya, people’s daily wage-earning struggles have already been intensified by lockdowns. Shelter-in-place is not an option for many residents of these communities: Their very livelihoods depend on informal work.

In anticipation of these consequences, Brazil’s government introduced emergency measures and plans to support informal workers with 600 Brazilian reais, around $114, each for three months. In Delhi, the authorities have appealed to employers to pay wages and landlords not to evict people. Clearly more is needed.

The Group of 20 has pledged $5 trillion in response to the coronavirus pandemic, which must include slum communities. Funds can be used to temporarily expand government conditional cash transfer programs, such as the Bolsa Família of Brazil and Prospera program of Mexico, which ran from the 1990s until February 2019. India has pledged to provide food rations and cash transfers for three months to the poor during the lockdown. Several countries have cash transfer programs designed to serve low-income populations; they must be expanded.

Displaced workers could even be trained and re-employed to conduct coronavirus contact-tracing in their own neighborhoods — along the lines of Brazil’s Family Health Strategy program, in which trained local residents provide preventive and basic primary care for their community.

Informal settlements are not limited to developing countries. The cities of high-income countries also house similar settlements. Homeless encampments in Los Angeles, Seattle, New York City, Oakland, Calif., Paris and London are vulnerable to the pandemic.

Over time, many of the world’s refugee camps have evolved into settlements that would meet the United Nations definition of slums. There are growing fears that refugee camps in Bangladesh, Lebanon, Kenya and Greece will not escape the coronavirus. At least 20 refugees living in a camp near Athens were diagnosed with Covid-19 last week.

The world’s public health systems and governments must make sure that people who live in slums, homeless encampments and refugee camps are not forgotten. We must prepare to deal with the consequences of the pandemic — for all populations.