Four years after a new coronavirus swept through New York in what was called a once-in-a-century event, public health officials are beginning to prepare for the possibility that a far worse pandemic is on its way.

The bird flu virus, H5N1, is not spreading among people. But the city is already preparing as if it could.

It is considering plans to set up isolation and quarantine hotels. One New York City hospital system is taking steps to start testing its sewage for the virus, so that it will know if bird flu is silently circulating among patients and staff.

But some epidemiologists worry that once again, the public health response in New York will be too sluggish in the early phases, should an epidemic break out in the city.

The virus has worried epidemiologists for its pandemic potential even before the first human patients were detected in Hong Kong in 1997. In the years since then, it has spread via migratory birds and periodically torn through chicken farms. Fewer than 1,000 people are known to have been infected in the past 20 years. But slightly more than half of those infected have died.

Now the disease has been found in herds of dairy cattle in 12 U.S. states. Human-to-human transmission has not yet been detected, but the worry is that the virus could evolve.