International alarm over the coronavirus that emerged in Wuhan, China, in December is driven by its rapid spread and the fact that infectious disease experts cannot yet know how deadly or contagious it is.
Within weeks, the virus has infected more people than Severe Acute Respiratory Syndrome (SARS) did in months. On Jan. 30, the World Health Organization declared the outbreak a global emergency.
The chart below shows the cumulative number of cases starting from the day that symptoms were documented for the first case. When compared to the new virus, the spread of SARS took much longer to gain momentum. Middle East Respiratory Syndrome (MERS) that first emerged in Saudi Arabia in 2012, took eight years to infect almost 2,500 people.
The latest statistics indicate a fatality rate of about 2.2%, but disease experts say the actual rate may be higher or lower as there are likely more unconfirmed cases.
The SARS virus killed about 10% of all infected individuals, while the MERS outbreak identified in 2012 had a fatality rate of around 35%.
Disease experts caution that it will take several more weeks to be confident of how the new virus behaves given how quickly it has spread and the fact that a reliable diagnostic test has only recently been introduced.
"Not everybody is being seen, not everybody is being tested," said Dr. William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center in Nashville.
"All the experts, myself included, tell the public that there is much we don't know about this virus and we are learning as we go along. That is not so reassuring."
Some experts question whether the new virus shares similarities with seasonal flu, which has a low mortality rate but infects so many people that more than half a million may die from it each year, according to global health estimates.
In emerging infectious disease outbreaks, the most serious cases are identified first. Coronavirus infections can range from mild cold-like symptoms to severe cases that cause pneumonia, acute respiratory illness and death.
About 20% of confirmed cases in the China coronavirus outbreak are classified as severe, similar to SARS and MERS, Schaffner said.