There is still a chance that Covid-19 will prove to be more fire drill than actual fire. A global pandemic is all but certain, but there are many unknowns: Will the virus itself prove to be less contagious or far less deadly than is currently feared? Will it show a tendency to recede in warmer weather the way that seasonal flu does? Can a vaccine be made quickly available? (Dr. Fauci says that one may be ready for testing in as little as two months, and could be commercially available in about a year.) Any of these developments may yet break the global transmission chain, and the vortex of fear and market-tumbling anxiety in which the world now finds itself may yet pass.

If the next few weeks or months bring calm — and scientists increasingly worry that they will not — the world would do well to remember this time what it seems to have forgotten again and again. Another pathogen will emerge soon enough, and another after that. Eventually, one of them will be far worse than all its predecessors. If we are very unlucky, it could be worse than anything in living memory. Imagine something as contagious as measles (which any given infected person passes to 90 percent of the people he or she encounters) only many times more deadly, and you’ll have a good sense of what keeps global health officials up at night.

Here’s what is certain: Despite many warnings over many years, we are still not ready. Not in China, where nearly two decades after that SARS outbreak food markets that sell live animals still thrive and authoritarianism still undermines honest and accurate communication about infectious diseases. Not in Africa, where basic public health capacity remains hobbled by a lack of investment and, in some cases, by political unrest and violence. Not in the United States, where shortsighted budget cuts and growing nationalism have shrunk commitments to pandemic preparedness, both at home and abroad.

To be sure, some broad progress has been made in the past few years. Vaccine development and deployment now proceed faster than at any point in history. The World Health Organization has corrected many of the institutional shortcomings that thwarted its responses to previous outbreaks. Other countries, in both Europe and Africa, have stepped up to fill the global health leadership position that America appears to have vacated.

But, as Covid-19 makes clear, much more is still needed.