As the new coronavirus spreads, doctors are seeing a pattern emerge in how people respond to the disease it causes, COVID-19.

At first, people experience mild symptoms such as a slight cough or a headache. After about a week, those symptoms can escalate to a trip to the emergency room.

One ICU doctor said COVID-19 patients often seem to be "doing OK, and then at around the five- to seven-day mark they start to get worse and then develop respiratory failure."

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As the novel coronavirus spreads, doctors are seeing a pattern in how patients respond to the disease it causes, COVID-19.

People who get sick tend to first experience minor ailments, such as a headache, a slight cough, and a slight fever, for about a week. But it's usually during the second week that they will either start improving or suddenly decline. For those who get worse, it can quickly escalate to a trip to the emergency room.

Among people who end up in the intensive-care unit, there's a similar delay in the way they display symptoms and relapse after receiving treatment.

Michelle Gong, the director of critical-care research at Montefiore Medical Center, said in a Q&A with the Journal of the American Medical Association on Monday that COVID-19 patients often seem to be "doing OK, and then at around the five- to seven-day mark they start to get worse and then develop respiratory failure."

The onset, she said, "can be very abrupt."

Many people who get COVID-19 feel fine for a week — and even seem to improve — then 'crash'

Dr. Joshua Denson, a critical-care physician in New Orleans, told NBC News over the weekend that based on the 15 to 20 COVID-19 patients he had treated, the first phase of illness was like "a slow burn."

Dr. Christopher Ohl, an infectious-diseases expert in North Carolina, also told NBC that he'd seen patients say they think they're getting better and "then within 20 to 24 hours they've got fevers, severe fatigue, worsening cough, and shortness of breath," and then "they get hospitalized."

The Centers for Disease Control and Prevention has said there is evidence that patients see a "clinical deterioration during the second week of illness."

A study published in The Lancet in late January found that more than half of patients developed shortness of breath after being ill for about a week.

"It's known as the second-week crash," Donald G. McNeil Jr., a science and health reporter for The New York Times, said on the podcast "The Daily" on Tuesday. "And some people crash even after they thought they were starting to get better."

Patients in the hospital seem to get better before they get worse

A respiratory therapist told ProPublica last week that "patients will be on minimal support, on a little bit of oxygen, and then all of a sudden they go into complete respiratory arrest, shut down, and can't breathe at all."

This sudden decline is more likely among elderly people and those with preexisting health conditions, but the respiratory therapist said he had seen it happen among healthy people in their 40s.

Unlike people with pneumonia or acute respiratory distress syndrome, a disease where fluid builds up in the lungs, people with COVID-19 often don't experience respiratory failure in the first few days of hospitalization, Gong said. She added that she had seen several COVID-19 patients with cardiac arrest, which might have to do with underlying conditions.

"But, again, it is sudden," Gong said. "It's not yet predictable to us as to who will get it and who will not."

Gong advised doctors not to use chloroquine, an immunosuppressive drug designed to treat malaria, for COVID-19 patients. She said that while she understood that physicians may be desperate for a cure, the drug could harm these patients, as the medical establishment still doesn't know exactly how it works on the illness — and it could deprive people who need the drug of their medicine.

"I still feel like we haven't seen the full extent," an internal-medicine doctor told Business Insider last week. "It's like leaning over the edge of a cliff."