Even mild coronavirus cases can be brutal. Here’s how to know when you should go to the hospital.

A nasal ventilator is pictured as a patient suffering from COVID-19 is treated in a pulmonology hospital in Vannes, France, March 20, 2020. Reuters/Stephane Mahe

For many coronavirus patients, the symptoms start out mild. On the first day or two, a typical patient might experience a fever, dry cough, shortness of breath, muscle soreness, or fatigue.

If symptoms worsen, they typically do so within five to 10 days.

On average, coronavirus patients in Wuhan and Wenzhou, China, who were admitted to the hospital went in after about a week of symptoms. Those with critical cases were admitted to the ICU about three days later.

Based on this research, doctors suspect that the first week of infection could be critical for determining whether a case is likely to become severe.

“We see that a lot of people stop shedding virus by day five to day seven – especially people who didn’t have more severe cases,” Megan Coffee, an infectious-disease clinician in New York City, told Business Insider. “Usually there’s control of the virus fairly early on in that first week.”

But severe cases can lead to respiratory failure – when the blood doesn’t receive enough oxygen. One way for doctors to flag these cases is to look for shortness of breath.

A recent study of 85 patients who died in Wuhan determined that the early onset of shortness of breath could be a warning sign of potentially fatal illness. Most of the patients in the study died from respiratory failure and two-thirds required machines to help them breathe.

Still, coronavirus patients are unlikely to have trouble breathing right away.

“If somebody is recording shortness of breath, that’s an important symptom,” Coffee said. “But that usually is what happens five, eight, 10 days into the course of the illness.”

Even then, not every patient with shortness of breath needs to go to the hospital. Coffee said patients can look for certain signs to determine whether they require immediate medical attention.

“Usually what I tell people is if you’re feeling short of breath and it’s hard to walk across the room, walk up a flight of stairs, speak in full sentences – those are always signs that you have more severe shortness of breath,” she said.

People who are having trouble inhaling and exhaling, she added, should go to the emergency room. But most patients with worsening symptoms should discuss their case with a doctor first.

“It’s really important to have a primary care doctor or someone you can talk to about your case so that you can decide whether you need to go to the emergency room or not,” Coffee said. “Right now we’re having to focus on everybody who needs intubation and critical care.”