“The war has changed.” That’s what the leaked C.D.C. document says about the way the Delta variant has upended our coronavirus policies. Delta is astonishingly contagious. It can generate 1,000 times the viral load of the original coronavirus strain, and it spreads with the ease of chickenpox. The vaccinated can no longer assume immunity. The unvaccinated are at more risk than ever. Masks are back. New York City is essentially imposing a vaccine mandate. I have so many questions about the war we’re now in. What do we actually know about Delta? If you’re vaccinated, is it more or less likely to kill you than the flu? Is it more serious for children? Are we re-masking to protect the unvaccinated, or is this also for the vaccinated? What are the risks of long Covid for the vaccinated? I could go on. Luckily, Dr. Céline Gounder has answers. Gounder is an epidemiologist at N.Y.U. medical school, a CNN medical analyst and host of the Covid podcast “Epidemic.” I’m not sure if this conversation will make you feel better about the war we’re now in. But it will, if nothing else, make it much, much clearer. Mentioned: "Improving Communications Around Vaccine Breakthrough and Vaccine Effectiveness" by Centers for Disease Control and Prevention Book recommendations: Wired for Culture by Mark Pagel Rule Makers, Rule Breakers by Michele Gelfand Stuck by Heidi J. Larson You can find transcripts (posted midday) and more episodes of "The Ezra Klein Show" at nytimes.com/ezra-klein-podcast, and you can find Ezra on Twitter @ezraklein. Thoughts? Guest suggestions? Email us at [email protected]. “The Ezra Klein Show” is produced by Annie Galvin, Jeff Geld and Rogé Karma; fact-checking by Michelle Harris; original music by Isaac Jones; mixing by Jeff Geld, audience strategy by Shannon Busta. Special thanks to Kristin Lin.