along and swallows packets with the leaf tissue. Once inside the caterpillar, a packet unfolds, sinister and orderly, like a MIRV warhead releasing its little nukes over a city. The virions disperse, attacking cells in the caterpillar’s gut. Each virion goes to the cell nucleus (again, hence the name), replicates abundantly, generating new virions that exit the cell and proceed to attack others. “They go from cell to cell, and infect lots and lots of cells,” Dwyer said. Before long the caterpillar is essentially just a crawling and eating bag of virus. Still, it doesn’t act sick. It doesn’t seem to know how sick it is. “If it has eaten a big enough dose,” he said, “then it will continue to wander around on leaves and continue to feed—but after maybe ten days, maybe two weeks, sometimes even as long as three weeks, it will melt onto a leaf.” There was that word again, the same one he had used in Atlanta, exquisitely vivid: melt.
Other caterpillars meanwhile are suffering the same fate. “The virus has almost completely consumed them before they really stop functioning.” Late in this process, as the virions within each caterpillar begin crowding one another, running short of food, they get themselves bundled together again within protective packets. Time to emerge. Time to move on. The caterpillar at this point is filled with virus, consumed by virus, held together only by its skin. But the skin, made of protein and carbohydrates, is tough and flexible. Then the virus releases certain enzymes, which dissolve the skin, and the caterpillar splits open like a water balloon. “They pick up the virus,” Dwyer said, and “they go splat on a leaf.” Each caterpillar disintegrates, leaving little more than a viral smudge—a smudge that, in the crowded conditions of an outbreak population of gypsy moths, is soon gobbled by the next hungry caterpillar. And so on. “Another insect comes along, feeds on that leaf, a week or two later,” Dwyer said, then repeated: “It goes splat.”
There might be five or six generations of splat in the course of the summer, five or six waves of transmission, with the virus progressively increasing its prevalence within the caterpillar population. From a starting point of low prevalence—say, 5 percent of the caterpillars are infected—it might grow to 40 percent by the first autumn. After the surviving caterpillars have metamorphosed to moths, and then mated, in a habitat still cluttered with NPV, some packets of the virus are left besmeared not just on foliage but on the egg masses laid by the female moths. So a large portion of the new caterpillars become infected the following spring as they hatch. The prevalence of the infection rises steeply. And that rise, beyond the preceding year’s level, “translates into an even higher percentage the following year,” Dwyer said. Within two or three years, such ratcheting “basically wipes out the entire population.”
The moths disappear and all that remains is the virus. Sometimes there’s so much of it, he added, that “you’ll see this kind of gray fluid trickling down the bark.” Rains come, and the trees weep with a slurry of dissolved caterpillars and virus. I was duly impressed.
It sounds like Ebola, I said.
“Yeah, right.” He had sat through some of the same meetings and read some of the same books and papers that I had.
Except not Ebola in reality, I said. The sensationalized Ebola, the popularized nightmare of Ebola, the hyped version of victims “bleeding out” like a sack of liquid guts.
He agreed. And the same distinction between degrees of gruesomeness, the real versus the exaggerated, applies to NPV. “With our virus, people like to say, they’ll say, ‘Oh, you study that virus that causes the insect to explode!’ Like, the virus doesn’t cause the insect to explode,” he insisted. “It causes it to melt.”
Having heard this scenario, and seen his graphs, and appreciated the directness of his language, and admired Maxwell’s equation on his T-shirt, I came to the point of my visit: what I called The Analogy. As of last week, I said, we’ve got 7 billion humans on this planet. It seems like an outbreak population. We live at high densities. Look at Hong Kong, look at Mumbai. We’re closely interconnected. We fly around. The 7 million people in Hong Kong are only three hours away from the 12 million people in Beijing. No other large animal has ever been as abundant. And we’ve also got our share of potentially devastating viruses. Some of