“Now, viruses are much, much smaller than bacteria, more difficult to detect, and much more mutable, hence difficult to cure,” Crusher was saying. She had conjured up six new images by now. “I’ve selected just a few examples that have plagued humans in the past…”

She highlighted each image as she identified it: “Herpes” was an orange, sponge-shaped orb with a spiked multicolored ring around it. “Polio” looked like nothing so much as an attractive blue-green sea anemone. “Smallpox” was a rusty-looking ovoid with an hourglass shape inside. “Hantavirus” looked like land masses on a planet, sickly pink, dotted with malevolent-looking little black seeds around the edges of each “continent.”

“Ebola,” Crusher continued. This looked like an aerial view of a series of crop circles in a wheat field. “Skorr pox.” This was a series of gray concentric hexagons. “And finally human immunodeficiency virus, or HIV.”

Glanced at quickly, this one was formless, a spider’s nest, a tuft of cat hair, something that might have rolled out from under the bed. But Crusher dismissed all the other images except the Gnawing, which still lingered on the periphery, and began to slowly enlarge the HIV virus. Gradually it became an elliptical shape with another shape inside it like an inverted tear drop, with a third, cylindrical shape within that. All of the shapes were studded about with strange artifacts that the readout identified as “surface glycoproteins,” “HLA I and II,” “core proteins (modified by AT-2).”

“I chose this one,” Crusher said, “Because it seems to most closely mimic the end stage of the bug we’re dealing with. Or, at least, one of the end stages.”

“One of them?” McCoy echoed her, scowling.

“I’ll get to that in a minute,” Crusher said.

“Go on,” Uhura encouraged her.

“Let’s take another look at our prime suspect,” she said. The HIV, rather than disappearing, simply moved slightly away from center stage at her instructions and the Gnawing bacillus moved in to hover beside it.

“I started with the specimens from the locket,” she explained. “There were four distinct compartments inside with blood, skin and hair samples from four victims. They were collected so meticulously I was able to classify them by gender and blood type. Whoever put this together was very skilled.”

She looked pointedly at Uhura. The question she wanted to ask was one Uhura still couldn’t answer. Were the samples themselves faked? Was this all a ploy to spread false rumors about an epidemic that didn’t exist, to divert Starfleet energies into pursuing a phantom, even to create an interstellar incident based on accusations of biological warfare? No way to answer any of that yet. Uhura wondered what progress Tuvok was making with Zetha.

“Go on,” she told Crusher.

“I grew each of these specimens in culture, and compared them with specimens from healthy Romulans kept in stasis on Starbase 23—and while we’re on the subject, I got a lot of flak when I requisitioned those. Mind telling me what we’re doing with Romulan blood specimens?”

“Left over from the Earth-Romulan War,” Uhura said tightly, watching Crusher’s and Selar’s eyebrows go up simultaneously. “And…other sources. On the rare occasion we’ve taken a Romulan prisoner alive, we take blood samples. They do the same with captured humans.”

She was avoiding McCoy’s eyes, though she could see him in her peripheral vision, scratching his stubble and looking uncomfortable. They both knew of at least three captured Romulans from their early years together, a commander accidentally beamed aboard Enterprise,and two of her guards who had started out as exchange prisoners but had not been returned until long after Enterprisehad beaten a hasty retreat out of the Neutral Zone with a stolen cloaking device.

“Go on,” she told Crusher again.

“When I compared the normals with the disease entities, the results were almost too good. There was the bug, all right. So I knew it would grow in vitro.I ran simulations based on several terrestrial and Vulcan life-forms.”

“Quite logical,” Selar commended her.

“Anyway,” Crusher went on. “There it was. But then I thought, ‘It can’t be this easy.’ And I was right. Because within two hours of regrowth, it had mutated into a viral form.”

Slowly she enlarged the Gnawing specimen image until the yellow-brown rods and vague blotches almost blotted out the empty space between, and some of the blotches showed bright green patches of something else. The entire entity moved, replicating inexorably as they watched, seeming almost to pulse malevolently.

“This is why it took me twenty-four hours to report,” Crusher explained. “Because we had to rule out the possibility that this might be a totally separate entity, so we observed isolated specimens of the bacillus until we actually caught them mutating into the viral form. What you’re looking at here is a time-lapsed version, ten hours compressed into less than a minute.” She froze the image. “And right about here seems to be the point at which the virus then mutates into a retrovirus.”

She paused. Her fellow MDs were looking grim. Uhura looked puzzled.

“Now I’m going to have to ask you to explain to me the difference between a virus and a retrovirus,” she said. “Use nice, simple words, please. As if you were explaining it to your son.”

The mention of Wesley, who’d just turned eleven, made Crusher smile.

“I think Wes is a lot more interested in physics than medicine. I guess most kids want to find themselves instead of following in their parents’ footsteps. Still, it isn’t really that complicated. A retrovirus is a virus that can infiltrate at the genetic level, become part of the patient’s DNA. HIV is a classic example. And I believe this thing is, too.”

“Okay,” Uhura said. “Still with you.”

“Now, we have the technology to not only identify every known virus, but to develop algorithms to identify unknowns. It might take a while, but we’d eventually catch the thing. Then we’d work backwards to create a decoy and—”

“A decoy,” Uhura repeated.

Crusher nodded. “I’ll use the HIV as an example, because it’s old news and we know exactly how it behaves and how to circumvent it: The virus attacks healthy cells by finding a way to get inside them and kill them. In the case of HIV, it does this by infiltrating a protein embedded in a T-cell membrane.

“T-cells are the Good Guys,” she said helpfully before Uhura could ask. “But HIV invaded the T-cells by attaching itself to the CD4 receptor located on the surface protein, and deactivated the T-cells. Without enough healthy T-cells, the patient had no defenses against a host of opportunistic infections and even certain cancers, and wasted away and died. With me so far?”

Not about to ask what a CD4 receptor was, Uhura nodded. “So far, yes.”

“Now, one way to distract the HIV virus and keep it from attacking the T-cells was to create a decoy cell. Decoy cells are genetically engineered molecules which look exactly like normal cells. They fool the virus into attaching to them instead of to the patient’s naturally occurring cells. The decoys grab onto the virus before it can do any harm, flush it out through the liver and kidneys and voilà.”

“So can we do that here?” Uhura nodded at the prime suspect, the Gnawing bacillus-turned-virus and, as if on cue, Crusher made the HIV disappear.

“We might,” Crusher said. “If this were only one virus.”

“Uh-oh,” McCoy murmured.

Crusher manipulated the image, creating a duplicate. As they watched, one model developed small splashes of orange, while the other continued replicating in green. When Crusher created a third image, it showed no growth at all and, in fact, the yellow-brown rods began to disappear. A fourth image showed no rods, but splashes of orange and green, interspersed with round gray nodules.