The green acolytes were another species of DBDG, differing only in detail from the furry Orligian DBDGs and clothes-wearing Earth-human DBDGs who made up most of Prilicla's crew. But the life-forms that appeared to be his patients were harder to classify. The Monitor Corps had asked for Sector General's help because the problem on this planet looked like a medical one. These large beings, who appeared to hold great religious or political power, suffered from a disease causing them to vomit periodically, and the Greens had taken it upon themselves to spend every spare moment helping their masters to eat. Frequently they went without food themselves, or fought and stole from one another to provide food for their sick masters.

The four thick, heavy-set beings were evidently warm-blooded oxygen-breathers, evolved to endure gravitational levels closer to those of Earth than of Prilicla's native Cinruss, which made them classification D-F – probably F, considering its heavy body-structure. They had no visible limbs, but a gaping mouth each, and skin which flickered with colourful patterns. As they sat in a row, awaiting their helpers, Prilicla sensed excitement and anticipation in them – and a hint of pleasurable rivalry, as if they were competing to see who could put on the most attractive display of colours. Far from appearing to suffer, they exuded well-being, and even what the Earth-humans called smugness.

As the acolytes formed polite queues to approach their four patients with food offerings, Prilicla was intrigued by their feelings, too. They didn't show the concern and sympathy that nurses on Sector General would have felt on, say, spraying nutrient-paint onto a sick Hudlar whose absorption-organ was malfunctioning. If anything, their greed and excitement mirrored those of their patients, as if they were expecting to receive gifts rather than give them. Perhaps the Monitor Corps had misunderstood, and there was a symbiotic relationship at work? But the Greens' excitement and hope was full of uncertainty, while the larger creatures seemed gleefully confident.

They shovelled food-offerings, mostly fruit or golden nectar, into the patients' mouth, much faster than looked healthy. The patients gobbled it eagerly, and flickered the patterns on their skins even more brightly than before as the acolytes stroked or tickled them for a few minutes, and then paused to see whether the acolytes had any more food. If there was any more, they had another course; if not, the acolyte would go on its way and another would take its place.

At first, the creatures ate with hearty appetites and apparently without mishap, until one suddenly vomited a stream of semi-digested fruit in syrup. The acolyte who had been feeding this creature, far from showing sympathy or concern, was plainly overjoyed as it held out its bucket for the patient to vomit into. Its emotions luminescent with triumph, it bent over and flipped its long tongue into the bucket, flicking its long tongue in and out as it lapped up a small portion of the syrup. Unlike the large, sleek patient, the Green was clearly half-starved, and relishing the first meal it had had in a long time. However, long before its hunger was satisfied, it tipped up the bucket to feed the rest of the vomit back to the patient again, and then resumed tickling the creature affectionately. After a few more minutes of tickling, it paused and waited hopefully, but this time the creature emitted only a contented burp. The acolyte was clearly disappointed, but still gave the creature one last, grateful pat before going on its way with its empty bucket.

As Prilicla watched, all four of the creatures continued to gorge themselves, vomit intermittently, and go on eating. He sensed that the periodic vomiting was not a result of indigestion, but an act they performed voluntarily when they deemed it prudent. Prilicla was an empath, not a telepath, but he was almost sure that the creatures were thinking, 'Better give them something occasionally, or they'll stop coming.' Equally, as he sensed the greed in the acolytes' minds, they seemed convinced that what they were doing was simply prudent, giving a little food in order to get a lot back. Yet, as far as Prilicla could see, the patients – or parasites? – were consuming vast amounts, and regurgitating very little.

Or was it the form of the food that made the arrangement worthwhile for the Greens? Perhaps the larger creatures took food that was inedible to Greens in its raw form, and turned it into something easier to digest? But that didn't explain why the Greens frequently simply gave back some of what had just been regurgitated.

Even stranger was the way Prilicla's memory-tapes were behaving. While his own Cinrusskin mind felt wary of the sessile creatures and concerned for their acolytes, and his Kelgian mind-partner wanted to bristle its fur in disgust, the human part of his mind made him wish that he, too, had brought some food for the pretty, colourful creatures. After all, they'd be good to him and feed him, if he only knew how to please them, and if these Greens hadn't been lucky, well, it was their own fault for not knowing how to win the favour of their wonderful benefactors…

Prilicla flew off as fast as he could, wishing his wings were large enough to do more in the heavy gravity of this planet. If these creatures could affect him simply because he was carrying an Earth-human tape, what might they be doing to his human friends?

Pathologist Murchison listened grimly as Prilicla explained what he had seen. 'I think I know what those are,' it said. 'I'd like to come with you to make sure, though. It's all right – as long as I'm not carrying any food, they won't be a threat to me.'

When they arrived back at the clearing, Murchison wagged its head up and down in the Earth-human gesture of confirmation. 'I thought so,' it said. 'Those parasites invaded my planet a few centuries ago. They enslaved us, starved us and broke up families, until we managed to drive them off. They're projective empaths, rather than receptive empaths like you, and they work by trying to convince us that all we want to do is feed them and stroke them.'

'Does approaching them closely intensify the danger?' asked Prilicla. 'If we're going to classify them, someone needs to pass a medical scan over them to understand their internal structure, and while I am, for evolutionary reasons, a complete coward, I might be less vulnerable than an Earth-human to attempts at deception.'

'You might,' said Murchison, 'but I already know the classification code, as it happens. It's FOBT.'

Author's note: I decided to write this story because there had been a lot of discussion in the news about the dangers of FOBTs (not, in this context, Faecal Occult Blood Testing), and I wondered what kind of creature an FOBT might be in the Sector General universe. So if you're confused, type 'FOBT' into a search engine, and follow the links that aren't about blood loss in the gastrointestinal tract.