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Across the US, prisons have become Covid-19 hotspots. Overcrowding, cramped quarters and antiquated facilities make social distancing extremely difficult, and curbing an outbreak nearly impossible.

And outbreaks in prisons can also lead to infections in neighbouring communities, according to a new study in Health Affairs.

In Illinois, researchers from Harvard University's Department of Anthropology and the Center National de la Recherche Scientifique in Paris found that more than 4,700 coronavirus cases up until mid-April were associated with 2,129 individuals going through Chicago's Cook County Jail in March.

The jail may be linked with more than 15% of all documented cases in the city and state, the researchers said.

"Existing conditions in jails and penitentiaries make infection control particularly difficult, putting inmates at unconscionable and perhaps unconstitutional risk," they wrote.

The Chicago jail was the largest recorded source of spread of the coronavirus before it was surpassed by an Ohio state prison, according to the researchers.

Read more about the spread of Covid in US prisons, and how one California institution went from zero cases in May to one of the worst outbreaks in the country.