Excerpt from Darwin's Ghost by Steve Jones, plus links to reviews, author biography & more

It was greeted with hysteria. Some claimed that the virus had been placed in Tutankhamen's tomb to punish those who defiled his grave and had come to America with an exhibition of his treasures. An analyst studied what he called its psycho-incubation. AIDS victims, he said, had suffered an emotional emergency in childhood that made them feel abandoned and later led to illness. The editor of Burke's Peerage went further. To preserve the purity of the human race his publication would not list any family in which a member was known to have the disease: "We are worried that AIDS may not be a simple infection, even if conveyed in an unusual way, but an indication of a genetic defect."

Although some dissenters tried to associate its symptoms with the use of capsules of amyl nitrate to enhance erotic pleasure, the real cause was soon found. The culprit is a virus, the human immunodeficiency virus, or HIV.

Like a whale, the virus is built on an inherited plan coded by genes, each one liable to accident every time it is copied. HIV is unusual even among viruses. As a retrovirus, its genes are based not on DNA, but on its relative RNA (a molecule used in most creatures to translate, rather than to transmit, the genetic message). All retroviruses-and they come in many forms-contain about ten thousand RNA units, or "bases." The AIDS virus subverts its host's cells. It forces them to make replicas of itself with an enzyme whose job is to copy information from the invader's RNA into human DNA. Each new particle hides itself in a cloak of cell membrane into which it inserts a protein. This is the key to the infection as it fits into matched molecules on the surface of blood cells and opens the door to their interior.

The lock that turns to an enemy's key is most abundant on certain cells of the immune system. These multiply in response to infection, but cannot cope with the challenge. Billions of new particles are made each day, and although most are at once destroyed, they soon prevail. Soon after the virus arrives, the number of protective cells falls, only to rise as the body's fight back begins. Then, the immune system begins to collapse. The first sign of illness is a malaise no worse than influenza. This clears up, but HIV stays at work. As the defenders are driven back, other diseases gain a hold. For most people, the transition from infection to overt illness takes from six to ten years.

As AIDS advances there may be pneumonia, fungal infections, diarrhea, weight loss and a viral form of blindness. A cancer called Kaposi's sarcoma, otherwise found among aged Jewish men, quite often appears. Its first sign is purple marks on the skin, but as it progresses it kills. Kaposi's sarcoma is caused by a herpes-like virus common in the homosexual community. It gains entry -- because the body's defenses have been undermined. If the patient does not first surrender to a fungus, bacterium or cancer, he wastes away.

The history of AIDS, over days, years and centuries, is simple. It involves descent, accompanied by modification. Each virus divides once a day. Mutation is followed by natural selection that allows the invader to adapt to the body's defenses, to the drugs used to treat it and to the sexual habits of the society in which it lives. Some changes are, it seems, unheeded by selection and build up at random as the generations pass. In time (and it does not take long) new forms of virus emerge.

The genes tell the story. They link a patient with the person who infected him, with others long dead, and with the viruses of apes, cats and whales. Except in its details, and the trivial matter of size, the evolution of the AIDS virus is that of every other being.

Excerpted from Darwin's Ghost by Steve Jones Copyright© 2000 by Steve Jones. Excerpted by permission of Random House, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.