Chinese scientists who looked at nearly 45,000 confirmed cases in the current COVID-19 outbreak concluded the death rate was 2.3%. But there are questions about whether all cases are being counted: Infected people with only mild symptoms may be missing from the tally. That means the true fatality rate may be lower.
Deadly is a broader concept that takes in how far and easily a virus spreads. SARS proved fatal in about 10% of cases in the 2003 outbreak but was controlled quickly and spread to about 8,000 people in all.
The flu's mortality rate is 0.1%, yet it kills hundreds of thousands around the world each year because it infects millions. So the size of the outbreak matters as much as the lethality in terms of how deadly a disease is.