Upon reinfecting T-cells, SVC would be capable of making an "informed decision" as to what the next generation would be. Like SVA, it would create a genetic fingerprint of its host cell. It would then compare this with its stored, ancestral copy. If the two fingerprints were identical - proving that the customised strain had remained within the body in which it had begun - its daughters would be, simply, more SVC.

 However, if the fingerprints failed to match, implying that the strain had now crossed into another person's body (and if gender-specific markers showed that the two hosts were not of the same sex), the daughter virus would be a third variety, SVM, containing both fingerprints. The M stood for "monogamous", or "marriage certificate". Shawcross, a great romantic, found it almost unbearably sweet to think of two people's love for each other being expressed in this way, deep down at the subcellular level, and of man and wife, by the very act of making love, signing a contract of faithfulness until death, literally in their own blood.

 SVM would be, externally, much like SVC. Of course, when it infected a T-cell it would check the host's fingerprint against both stored copies, and if either one matched, all would be well, and more SVM would be produced.

 Shawcross called the fourth form of the virus SVD. It could arise in two ways; from SVC directly, when the gender markers implied that a homosexual act had taken place, or from SVM, when the detection of a third genetic fingerprint suggested that the molecular marriage contract had been violated.

 SVD forced its host cells to secrete enzymes that catalysed the disintegration of vital structural proteins in blood vessel walls. Sufferers from an SVD infection would undergo massive haemorrhaging all over their body. Shawcross had found that mice died within two or three minutes of an injection of pre-infected lymphocytes, and rabbits within five or six minutes; the timing varied slightly, depending on the choice of injection site.

 SVD was designed so that its protein coat would degrade in air, or in solutions outside a narrow range of temperature and pH, and its RNA alone was non-infectious. Catching SVD from a dying victim would be almost impossible. Because of the swiftness of death, an adulterer would have no time to infect their innocent spouse. The widow or widower would, of course, be sentenced to a life of celibacy, but Shawcross did not think this too harsh: it took two people to make a marriage, he reasoned, and some small share of the blame could always be apportioned to the other partner.

 Even assuming that the virus fulfilled its design goals precisely, Shawcross acknowledged a number of complications:

 Blood transfusions would become impractical until a foolproof method of killing the virus in vitro was found. Five years ago this would have been tragic, but Shawcross was encouraged by the latest work in synthetic and cultured blood components, and had no doubt that his epidemic would cause more funds and manpower to be diverted into the area. Transplants were less easily dealt with, but Shawcross thought them somewhat frivolous anyway, an expensive and rarely justifiable use of scarce resources.

 Doctors, nurses, dentists, paramedics, police, undertakers . . . well, in fact everyone, would have to take extreme precautions to avoid exposure to other people's blood. Shawcross was impressed, though of course not surprised, at God's foresight here: the rarer and less deadly AIDS virus had gone before, encouraging practices verging on the paranoid in dozens of professions, multiplying rubber glove sales by orders of magnitude. Now the overkill would all be justified, since everyone would be infected with, at the very least, SVC.

 Rape of virgin by virgin would become a sort of biological shot-gun wedding; any other kind would be murder and suicide. The death of the victim would be tragic, of course, but the near-certain death of the rapist would surely be an overwhelming deterrent. Shawcross decided that the crime would virtually disappear.

 Homosexual incest between identical twins would escape punishment, since the virus could have no way of telling one from the other. This omission irritated Shawcross, especially since he was unable to find any published statistics that would allow him to judge the prevalence of such abominable behaviour. In the end he decided that this minor flaw would constitute a necessary, token remnant - a kind of moral fossil - of man's inalienable potential to consciously choose evil.

It was in the northern summer of 2000 that the virus was completed, and tested as well as it could be in tissue culture experiments and on laboratory animals. Apart from establishing the fatality of SVD (created by test-tube simulations of human sins of the flesh), rats, mice and rabbits were of little value, because so much of the virus's behaviour was tied up in its interaction with the human genome. In cultured human cell lines, though, the clockwork all seemed to unwind, exactly as far, and never further, than appropriate to the circumstances; generation after generation of SVA, SVC and SVM remained stable and benign. Of course more experiments could have been done, more time put aside to ponder the consequences, but that would have been the case regardless.

 It was time to act. The latest drugs meant that AIDS was now rarely fatal - at least, not to those who could afford the treatment. The third millenium was fast approaching, a symbolic opportunity not to be ignored. Shawcross was doing God's work; what need did he have for quality control? True, he was an imperfect human instrument in God's hands, and at every stage of the task he had blundered and failed a dozen times before achieving perfection, but that was in the laboratory, where mistakes could be discovered and rectified easily. Surely God would never permit anything less than an infallible virus, His will made RNA, out into the world.

 So Shawcross visited a travel agent, then infected himself with SVA.

Shawcross went west, crossing the Pacific at once, saving his own continent for last. He stuck to large population centres: Tokyo, Beijing, Seoul, Bangkok, Manilla, Sydney, New Delhi, Cairo. SVA could survive indefinitely, dormant but potentially infectious, on any surface that wasn't intentionally sterilised. The seats in a jet, the furniture in a hotel room, aren't autoclaved too often.

 Shawcross didn't visit prostitutes; it was SVA that he wanted to spread, and SVA was not a venereal disease. Instead, he simply played the tourist, sight-seeing, shopping, catching public transport, swimming in hotel pools. He relaxed at a frantic pace, adopting a schedule of remorseless recreation that, he soon felt, only divine intervention sustained.

 Not surprisingly, by the time he reached London he was a wreck, a suntanned zombie in a fading floral shirt, with eyes as glazed as the multicoated lens of his obligatory (if filmless) camera. Tiredness, jet lag, and endless changes of cuisine and surroundings (paradoxically made worse by an underlying, glutinous monotony to be found in food and cities alike), had all worked together to slowly drag him down into a muddy, dreamlike state of mind. He dreamt of airports and hotels and jets, and woke in the same places, unable to distinguish between memories and dreams.

 His faith held out through it all, of course, invulnerably axiomatic, but he worried nonetheless. High altitude jet travel meant extra exposure to cosmic rays; could he be certain that the virus's mechanisms for self checking and mutation repair were fail-safe? God would be watching over all the trillions of replications, but still, he would feel better when he was home again, and could test the strain he'd been carrying for any evidence of defects.