Because the tunnel was man-made and contained few other species, it was a perfect testing ground for this agent. The tunnel’s tricolored bat population numbered over 5,000 in 2013 but was down to 152 in a count last year.

“If the numbers are too low today, we may have to abandon the project,” Pete Pattavina, a bat biologist for the federal government, told me as we drove to the site. “It’s hard to justify the continued expense if we only have a few survivors.”

Not everyone thinks the efforts are worthwhile. The National Speleological Society, dismayed by the closing of caves on federal land in what it sees as a misguided effort to prevent the spread of white-nose syndrome, thinks the war has been lost, that the government efforts are putting bats at greater risk and that, in any event, the disease is going to run its course. This may be what happened in Europe, where bats appear have developed resistance to it.

I don’t agree. While targeted cures like the one being tested in Black Diamond would likely be impractical for most wild caves, they could potentially slow the leading edges of the disease. Kelly Lutsch, a first-year graduate student of Dr. Cornelison, has been surveying concrete culverts along Interstates 75 and 95 through Georgia’s coastal plains. She believes that infected bats using the culverts could be carrying the pathogen south into cave-rich zones near the Florida panhandle. If so, spraying the culverts with B23 might be warranted. One Georgia cave near the Florida border hosts a robust colony of 90,000 southeastern myotis bats.

At the Black Diamond Tunnel, the results of the spraying were encouraging. After an hour, the team slowly paddled back to share its results: 178 bats, up by 26 from the last count.

Dr. Cornelison cautioned against interpreting this as a success for B23, since other factors could affect the count. Still, he declared himself “ecstatic” that the number was higher, not lower, and plans to test B23 again in the tunnel.