There are two reasons to be legitimately concerned about the COVID-19 (COrona VIrus Disease 2019) virus: its mortality rate and its incubation period. You’re Likely to Get the Coronavirus (Atlantic). Because it is novel, the actual mortality rate is presently unknown (although in the vicinity of 2% - in contrast to influenza, which is typically 0.1-.2% ). The tests for determining exposure are not particularly robust or widespread, which is a worldwide problem. That is why the case in California was so alarming. That patient was undiagnosed for a week after exhibiting severe symptoms.

Influenza typically hits between 9 and 14 million Americans annually (3-4% of the population), and 12-56,000 Americans die from it annually. And that is with a robust and relatively effective inoculation program. It has an incubation period of 1-4 days. More than 80 percent of infections result in mild to no symptoms, which is why it spreads so widely.

The coronavirus incubation period appears to be between 14-22 days. So, COVID-19 is 10-20 times more virulent than influenza, and has 3-20 times the incubation period. To put that in perspective given the spread rate of influenza in the population,
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within the coming year, some 40 to 70 percent of people around the world will be infected with the virus that causes COVID-19. But, [Harvard epidemiology professor Marc Lipsitch] clarifies emphatically, this does not mean that all will have severe illnesses. “It’s likely that many will have mild disease, or may be asymptomatic,” he said. As with influenza, which is often life-threatening to people with chronic health conditions and of older age, most cases pass without medical care. (Overall, about 14 percent of people with influenza have no symptoms.)
All of this is to explain why this pandemic is so alarming to epidemiologists. When you map it out, it is, conservatively, perhaps 10 times as deadly as influenza, and can be 10-15 times more widespread. And, there is no vaccine (nor likely to be one this year). Extrapolating it out, that would be (again, conservatively), 132 million American infections, and 2.64 million deaths. That is a sobering picture.

Coronavirus Disease 2019 (COVID-19) (CDC):
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Outbreaks of novel virus infections among people are always of public health concern. The risk from these outbreaks depends on characteristics of the virus, including how well it spreads between people, the severity of resulting illness, and the medical or other measures available to control the impact of the virus (for example, vaccine or treatment medications). The fact that this disease has caused illness, including illness resulting in death, and sustained person-to-person spread is concerning. These factors meet two of the criteria of a pandemic. As community spread is detected in more and more countries, the world moves closer toward meeting the third criteria, worldwide spread of the new virus.

The potential public health threat posed by COVID-19 is high, both globally and to the United States.