Last Updated Mar 2, 2020 2:42 PM EST
As cases of coronavirus spread around the world, doctors are learning more about . Most cases are mild, but the illness — officially known as COVID-19 — can become severe if a person's immune system cannot repair damage to the lungs caused by the virus, CBS News medical contributor Dr. Tara Narula said. The disease has infected almost 89,000 people worldwide and killed more than 3,000 people. In the United States, the death toll rose to six on Monday.
"The issue with COVID-19 is that this virus can affect the lower tract of your airways," Narula, a cardiologist at Northwell Health, said Monday on "CBS This Morning." The virus can damage the cells that line the respiratory tract in the lungs, she explained. When that happens, "your immune system launches a response to try to clean up and repair."
But for some people, the immune system response "is so overwhelming, it's not in check," Narula said. If that's the case, the lungs can be flooded with fluid and cellular debris.
"Essentially the lungs start to drown, and that's a situation that can become a severe pneumonia. The pneumonia can then progress to what we call, where you can have a drop of blood pressure and multi-organ failure, and that's how it really causes death," she said.
In China, where the virus originated, 80% of cases have been mild, Narula said.
"Only about 14% were severe and even less were critical," she said.
Some research has estimated the death rate at about 2%, Narula said, but added that it's probably lower because there are likely more mild cases than what have been detected.
"In the grand scheme of things, for most people if they get it, it will end up being a mild disorder," she said.
Narula said the virus is transmitted through person-to-person contact by droplets.
"So if I cough or sneeze, those droplets can travel up to six feet and enter your mouth, or your nose or your lungs," she said. "Another way is if I cough or sneeze and it lands on a surface or an object and then you touch that same object or surface and touch your face."
Symptoms include fever, cough and shortness of breath. People could also have a headache, diarrhea, muscle aches, sore throat, runny nose and congestion, Narula said.
An infected person can pass on the virus to others even if they are not showing any symptoms.