is this the worst outbreak since the spanish flu and the plague?
Not yet, and in the end it'll probably depend on what metric you use to compare things.
If we use the most serious of metrics (number of deaths) then there is a vast gap between where we are now with Covid-19 (approaching 5,000 worldwide) and the diseases you mentioned (I assume you're specifically referring to the "Black Death" plague since that's the most widely known). Both your examples have tens and maybe hundreds of millions of deaths. They're even worse per capita - the Black Death is generally estimated to have killed around a third of Europe, which would have meant that every survivor was likely to have lost several family members, many friends and dozens of acquaintances. From their perspective it was a universe-altering event. Even the worst projections of Covid-19 don't suggest anything like that is plausible.
Other outbreaks since the Spanish Flu are also currently worse than Covid-19, based on deaths. Several flu outbreaks killed over a million people. Several more localised diseases also resulted in more deaths, such as Ebola in 2014 and Cholera in Haiti after 2010, following the earthquake there. Both of these will, however, be overtaken by Covid-19.
Additionally, Covid-19 is one of two diseases currently classified as a pandemic. The other is HIV/AIDs - it is estimated to have killed over 30m people - primarily in sub-Saharan Africa where it resulted in severely reduced life expectancy levels in the 1990s.
So right now, Covid-19 is not the worst outbreak of disease since the Spanish Flu. It is likely to be among the worst, however, when it can eventually be assessed in retrospect.
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www.worldometers.info
