'Really?' said Urmila, her forehead wrinkling in disbelief. 'Is it possible that she could have taught herself something as technical as that?'

Murugan shrugged. 'Similar things have been known to happen,' he said. 'Think of Ramanujan, the mathematician, down in Madras. He went ahead and reinvented a fair hunk of modern mathematics just because nobody had told him that it had already been done. And with Mangala we're not talking about mathematics: we're talking about microscopy, which was still an artisanal kind of skill at that time. Real talent could take you a long way in it – Ronnie Ross's career is living proof of that. With this woman we're talking about a whole lot more than just talent; we may be talking genius here. You also have to remember that she wasn't hampered by the sort of stuff that might slow down someone who was conventionally trained: she wasn't carrying a shit-load of theory in her head, she didn't have to write papers or construct proofs. Unlike Ross she didn't need to read a zoological study to see that there was a difference between culex and anopheles: she'd have seen it like you or I can see the difference between a dachshund and a Dobermann. She didn't care about formal classifications. In fact she didn't even really care about malaria. That's probably why she got behind Ronnie Ross and started pushing him towards the finish line. She was working towards something altogether different, and she'd begun to believe that the only way she was going to make her final breakthrough was by getting Ronnie Ross to make his. She had bigger things in mind than the malaria bug.'

'Like what?' said Urmila.

'The Calcutta chromosome.'

With a discreet cough, the waiter parted their curtain, and placed their orders on the table.

Urmila waited till he had left. 'What was that you just said?'

'The Calcutta chromosome,' said Murugan. 'That's my name for what she was working towards.'

'Now I'm really lost,' said Urmila. 'I've lived here all my life and I've never heard of this thing you're talking about.'

'And who knows if you ever will?' said Murugan. 'Or whether I will. Or whether it exists or has ever existed. At this point in time it's still all guesswork on my part.'

'But you must have something to found your guesses on,' said Urmila.

Murugan made no answer. 'Go on,' Urmila prompted, almost pleading. 'We're caught in this together, after all. I have a right to know.'

Murugan hesitated. 'Are you really sure you want to know?'

She nodded.

'OK, I'll tell you what started me off,' said Murugan reluctantly. 'It seemed to me from Farley's letter that Mangala was actually using the malaria bug as a treatment in another disease.'

'What disease?'

'Syphilis,' said Murugan. 'Or to put it more precisely, syphilitic paresis – the final paralytic stage of syphilis. From Farley's account it seems there was an underground network of people who believed that she possessed a cure. Remember that we're talking about the 1890s – long before the discovery of penicillin. Syphilis was untreatable and incurable: it killed millions of people every year, all around the world. These people who came to see Mangala may have believed that she was a witch or a magician or a god or whatever: it doesn't matter – the conventional medical treatments for syphilis at that time weren't much more than hocus-pocus either. Let's just stick with that old saying about no smoke without a fire. If a whole crowd of people believed that Mangala had a cure, or a halfway effective treatment, it must have been because she had a certain rate of success. People aren't crazy: if they travelled long distances to see her they must have thought she offered some kind of hope.'

'What do you think that treatment was?' said Urmila.

'I'm just guessing wildly here, OK? But if you twisted my arm, I'd say that she'd stumbled upon some variant of a process that got a guy called Julius Wagner-Jauregg the Nobel in 1927. Guess what that treatment was?'

Urmila looked up from her plate. 'You know perfectly well I have no idea,' she said. 'What was it?'

Murugan poked a finger into the crisp, rounded top of his dhakai parotha, releasing a puff of steam. 'All right, I'll tell you,' he said. 'What Wagner-Jauregg showed was that artificially induced malaria often cured, or at least mitigated, syphilitic paresis. What he'd do is, he'd actually inject malarial blood into the patient by making a little incision. It was a pretty crude process, but the weird thing is that it worked. In fact, until antibiotics, the WagnerJauregg process was pretty much a standard treatment: every major VD hospital had its little incubating room where it grew a flock of anopheles. Think about it: hospitals cultivating disease! But on the other hand, what could be more natural than fighting fire with fire? You could say vaccines work on the same sort of principle really, but what they do is to prime your immune system against themselves. This is the only instance known to medicine of using one disease to fight another.

'To this day no one really knows how the WagnerJauregg treatment worked. Not that anyone's losing any sleep over it. It was a scientific scandal and medicine was almost grateful to turn its back on it once antibiotics came along. Old Julius didn't worry too much about how it worked either. He was no biologist, remember: he was a clinician and a psychologist. He thought the process worked by raising the patient's body temperature. It didn't seem to bother him that no other fever had the same effect.

'But it's quite possible that malaria worked on paresis through a different route: the brain, for example. One of the things that syphilis does is that it muddies up the blood! brain barrier. Malaria works on the brain too, in different ways: that's why falciparum malaria is also called cerebral malaria. But other kinds of malaria have weird neural effects too. A lot of people who've had malaria know that: it can be more hallucinogenic than any mind-bending drug. That's why primitive people sometimes thought of malaria as a kind of spirit-possession.

'Enter Mangala: it looks like she hit upon this treatment too, at about the same time as the Herr Doktor. But she added a little twist to it. From what we know of her technique, it sounds like she was working with some weird strain of malaria – that is, by some kind of primitive horsebreeding method she had developed a strain that could actually be cultivated in pigeons. My hunch is that she found some way of making the bug cross over, so that the bird could be used like a test tube, or an agar plate.

'Now here's the really wacky stuff. I'll just stick my neck out and say it: I think what happened was that somewhere down the line Mangala began to notice that her treatment often produced weird side effects – what looked like strange personality disorders. Except that they weren't really disorders but transpositions. She began to put two and two together and found that in fact what she had on her hands was a crossover of randomly assorted personality traits, from the malaria donor to the recipient – via the bird of course. And once she saw this she became more and more invested in isolating this aspect of the treatment, so that she could control the ways in which these crossovers worked.'

'I'm not sure I follow,' said Urmila. 'What exactly are you trying to say?'

'What am I saying? Well, what I'm saying is this: I think Mangala stumbled on something that neither she nor Ronnie Ross nor any scientist of that time would have had a name for. For the sake of argument let's call it a chromosome: though the whole point of this is that if it is really a chromosome, it's only so by extension, so to speak – by analogy. Because what we're talking about here is an item that is to the standard Mendelian pantheon of twenty-three chromosomes what Ganesh is to the gods; that is, different, non-standard, unique – which is exactly why it eludes standard techniques of research. And which is why I call it the Calcutta chromosome.