The COVID-19 pandemic may be the deadliest viral outbreak the world has seen in more than a century. But statistically, such extreme events aren't as rare as we may think, asserts a new analysis of novel disease outbreaks over the past 400 years. A newly assembled record of past outbreaks was used to estimate the probability of a pandemic with similar impact to COVID-19 is about 2 percent in any year. And that probability is growing.