Some individuals infected with the new coronavirus have no symptoms, but they can still transmit the virus—a phenomenon that presents a new obstacle to researchers that are trying to curb transmission of the virus. 

Our analysis: The 'recurring themes' of disease outbreaks

Background: Coronavirus cases spike outside of China

Reports of the new coronavirus first surfaced in early December 2019 in Wuhan, China. According to the World Health Organization (WHO), the main symptoms of the virus are fever and lesions in both lungs. Some patients also have reported difficulty breathing, WHO said.

As of Monday, officials reported more than 89,700 cases of the virus globally, with most of those cases occurring in mainland China, the New York Times reports. Officials said as of Monday there had been at least 3,056 deaths linked to the virus, and all but 144 of the deaths occurred in mainland China.

In the United States, officials as of Monday had reported a total of 88 cases of the virus and two deaths linked to the virus, the Times reports.

Why the asymptomatic coronavirus cases matter

Researchers are still learning about the new virus and how it transmits, but recent evidence suggests some people infected with the virus are asymptomatic but can still be contagious, the Times reports.

An early report of asymptomatic transmission surfaced in January when a Chinese woman visited Germany and infected several colleagues before she realized she was ill. A follow-up report found the woman had "vague symptoms" but no symptoms that have been associated with the virus.

And in February, after German officials evacuated 126 people from Wuhan, two people who reported zero symptoms of the virus tested positive.

Experts say asymptomatic people can spread viruses efficiently. They have no reason to think they're sick, so they rarely change their behavior to prevent transmission, according to the Times.

Until Thursday, CDC was only performing coronavirus testing only on symptomatic patients who'd either recently traveled to China or had contact with someone who had the virus. CDC last week updated that criteria to include symptomatic patients with no known connection to China or a person already diagnosed with the virus.

Sandra Ciesek from the Institute of Medical Virology at University Hospital Frankfurt, said, "[N]ormally, you don't screen asymptomatic healthy people for the virus because it's too expensive."

However, health experts said if research shows that asymptomatic carriers can spread the virus efficiently and quickly, testing of the virus might need to be broadened.

Mild cases make up majority of coronavirus cases

For the majority of cases, patients do show symptoms of the virus, but these symptoms tend to be mild. According to Jin Dongyan from the University of Hong Kong, most mild cases of the virus are indistinguishable from a common cold. Other symptoms can include mild fatigue and a low fever, according to Chinese officials.

The majority of patients with mild cases of the virus recover, but experts say the mild symptoms can make the virus more difficult to contain because, like asymptomatic patients, patients with mild cases might not know they're carrying the virus.

A study of more than 44,600 cases confirmed in China by Feb. 11 found more than 81% were mild. For the study, published by China's Center for Disease Control and Prevention, cases were considered mild if a patient did not have pneumonia or only experienced mild pneumonia. In the same study, less than 14% of cases were severe and less than 5% were critical. Severe cases were defined as shortness of breath, low blood oxygen saturation, or other lung problem, while critical cases meant the patient suffered respiratory failure, septic shock, or multiple organ dysfunction.

But the fact that most cases are mild doesn't mean the virus isn't a threat, according to the Times.

Regarding the severity of the illness, a group of scientists wrote in a piece published in the New England Journal of Medicine last week wrote, "In this manner, a virus that poses a low health threat on the individual level can pose a high risk on the population level, with the potential to cause disruptions of global public health systems and economic losses" (Caryn Rabin, New York Times, 2/26; Wang, New York Times, 2/27; Lai et al., New York Times, 3/2).