The Centers for Disease Control and Prevention has expanded its list of possible symptoms of the coronavirus, a step that reflects the broad variation and unpredictability in the way the illness can affect individual patients.

Echoing the observations of doctors treating thousands of patients in the pandemic, the federal health agency this month changed its website to cite the following symptoms as possible indicators of COVID-19, the infection caused by the coronavirus: chills, repeated shaking with chills, muscle pain, headache, sore throat and new loss of taste or smell.

Previously it had listed just three symptoms: fever, cough and shortness of breath.

The CDC made no public announcement when it added the six new symptoms to its website April 18, and the agency did not immediately respond to questions about the revised list.

While people who become seriously ill from coronavirus infection primarily have acute respiratory distress, other symptoms that accompany the disease can vary widely, doctors and researchers have reported.

It has turned out, for example, that many people with COVID-19 do not have fevers or that their fevers wax and wane, and are sometimes accompanied by chills.

Shortness of breath can emerge at the same time as other symptoms or it can crop up suddenly a week or even 10 days after a person has been experiencing more manageable symptoms like cough and aches.

Some people report a notable loss of smell and taste, an effect that can also occur with other respiratory infections.

The revised CDC list differs somewhat from the symptoms described by the World Health Organization on its website. The WHO says the most common symptoms are fever, dry cough and tiredness.

“Some patients may have aches and pains, nasal congestion, sore throat or diarrhea,” the WHO says. “These symptoms are usually mild and begin gradually.”

Both agencies recommend that people seek emergency medical attention if they have trouble breathing or persistent pain or pressure in the chest. The CDC also says that immediate medical attention should be sought if people experience sudden confusion, cannot be aroused or if their lips or face turn a bluish color.

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