Editor's Note: Luc Montagnier shared the 2008 Nobel Prize in Medicine or Physiology, awarded on October 6. The new Nobel laureate co-authored this article, originally published in the October 1988 issue of Scientific American. We are making it available here due to its historical significance.

As recently as a decade ago it was widely believed that infectious disease was no longer much of a threat in the developed world. The remaining challenges to public health there, it was thought, stemmed from noninfectious conditions such as cancer, heart disease and degenerative diseases. That confidence was shattered in the early 1980's by the advent of AIDS. Here was a devastating disease caused by a class of infectious agents--retroviruses--that had first been found in human beings only a few years before. In spite of the startling nature of the epidemic, science responded quickly. In the two years from mid-1982 to mid-1984 the outlines of the epidemic were clarified, a new virus-the human immunodeficiency virus (HN)-was isolated and shown to cause the disease, a blood test was formulated and the virus's targets in the body were established.

Following that initial burst, progress has been steady, albeit slower. Yet in some respects the virus has outpaced science. No cure or vaccine is yet available, and the epidemic continues to spread; disease-causing retroviruses will be among the human population for a long time. In view of that prospect, it is essential to ask where we stand in relation to AIDS in 1988. How was HN discovered and linked to AIDS? How does the virus cause its devastation? What are the chances that AIDS will spread rapidly outside the known high-risk groups? What are the prospects for a vaccine? For therapy? How can the epidemic most effectively be fought? Those are some of the questions this article and this issue of Scientific American have set out to answer.

Like other viruses, retroviruses cannot replicate- without taking over the biosynthetic apparatus of a cell and exploiting it for their own ends. What is unique about retroviruses is their capacity to reverse the ordinary flow of genetic information--from DNA to RNA to proteins (which are the cell's structural and functional molecules). The genetic material of a retrovirus is RNA In addition, the retrovirus carries an enzyme called reverse transcriptase, which can use the viral RNA as a template for making DNA The viral DNA can integrate itself into the genome (the complement of genetic information) of the host. Having made itself at home among the host's genes, the viral DNA remains latent until it is activated to make new virus particles. The latent DNA can also initiate the process that leads to tumor formation.

Retroviruses and their cancer causing potential are not new to science. At the beginning of this century several investigators identified transmissible agents in animals that were capable of causing leukemias (cancers of blood cells) as well as solid-tissue tumors. In the succeeding decades retroviruses were identified in many animal species. Yet the life cycle of retroviruses remained obscure until 1970, when Howard M. Temin of the University of Wisconsin at Madison and (independently) David Baltimore of the Massachusetts Institute of Technology discovered reverse transcriptase, confirming Temin's hypothesis that the retroviral life cycle includes an intermediate DNA form, which Temin had called the provirus. The details of viral replication quickly fell into place.

In spite of such discoveries, by the mid-1970's no infectious retroviruses had been found in human beings, and many investigators firmly believed no human retrovirus would ever be found. Their skepticism had several grounds. Many excellent scientists had tried and failed to find such a virus. Moreover, most animal retroviruses had been fairly easy to find, because they replicated in large quantities, and the new virus particles were readily observed in the electron microscope; no such phenomenon had been found in human beings. In spite of this skepticism, by 1980 a prolonged team effort led by one of us (Gallo) paid off in the isolation of the first human retrovirus: human T-lymphotropic virus type I (HTLV-I).

HTLV-I infects T lymphocytes, white blood cells that have a central role in the immune response. The virus causes a rare, highly malignant cancer called adult T-cell leukemia (ATL) that is endemic in parts of Japan, Africa and the Caribbean but is spreading to other regions as well. Two years after the discovery of HlLV-I the same group isolated its close relative, HTLVII. HTLV-II probably causes some cases of a disease called hairy-cell leukemia as well as T-cell leukemias and lymphomas of a more chronic type than those linked to HTLV-I. The two viruses. however. share some crucial features. They are spread by blood. by sexual intercourse and from mother to child. Both cause disease after a long latency. and both infect T lymphocytes. When AIDS was first recognized. these properties took on great additional significance.

The first AIDS cases were diagnosed in 1981 among young homosexual men in the U.S.  Although the syndrome was puzzling. it soon became clear that all its victims suffered from a depletion of a specific subset of T cells- T4 cells and that as a result they fell prey to pathogens that would easily be controlled by a healthy immune system. A variety of hypotheses were advanced to explain AIDS. including breakdown of the victims' immune systems following repeated exposure to foreign proteins or even to sperm-during homosexual intercourse. It seemed more plausible. however. to explain a new syndrome by the appearance of a new infectious agent.

To one of us (Gallo) the likeliest agent was a retrovirus. It had already been shown that the AIDS pathogen. like HTLV-I. could be transmitted by sexual intercourse and by blood. Furthermore. Max Essex of the Harvard School of Public Health had shown that a retrovirus of cats called feline leukemia virus (FeLV) could cause either cancer or immune suppression. Since in most species the infectious retroviruses are closely related. it seemed plausible that the same was true in human beings. Hence the initial hypothesis was that the cause of AIDS was a close relative of HTLV-I. That hypothesis. as it turned out. was wrong. Nonetheless, it was fruitful because it stimulated the search that led to the correct solution.

The retrovirus hypothesis for the origin of AIDS reached the other one of us in France in the following way. Almost as soon as AIDS was first diagnosed. a working group on the syndrome had been formed by a circle of young clinicians and researchers in France. One member of the group. Jacques Leibowitch of the Raymond Poincare Hospital in Paris. had had some contact with Gallo's team and carried the HTLV hypothesis back to France. The members of the French group wanted to test that hypothesis. and they had the biological materials to do so because the group included clinicians with patients afflicted by AIDS or pre-AIDS. What they lacked, however, was the collaboration of virologists experienced in work with retroviruses.

The French author of this article and his colleagues Francoise Barre-Sinoussi and Jean-Claude Chermann at the Pasteur Institute fitted that description. They were engaged in several lines of work on cancer and interferon including attempts to find retroviruses in patients with cancer particularly in cultures of lymphocytes. A member of the working group, Willy Rozenbaum of the Salpetriere Hospital, asked whether they were interested in analyzing tissues from a patient with lymphadenopathy. or swollen glands. (Lymphadenopathy can be an early sign of the process that culminates in AIDS. Such a patient was chosen because finding a virus early in the disease seemed more meaningful than finding one later. when AIDS patients were infected with many opportunistic agents.) The answer was yes, and in January, 1983, a specimen from the swollen lymph node of a young homosexual arrived at Montagnier's laboratory.

The specimen was minced. put into tissue culture and analyzed for reverse transcriptase. After two weeks of culture. reverse-transcriptase activity was detected in the culture medium. A retrovirus was present. But which one? The first possibility that had to be tested was whether the virus was one of the known HTLVs. or perhaps a close relative of them. That possibility was tested using specific HTLV-I reagents supplied by Gallo. The virus did not react significantly with the HTLV-I reagents; a similar result was later obtained with HTLV-Il reagents. A strenuous effort was begun to characterize the new agent.

Among the first results of that effort was the finding that the new virus (which was named lymphadenopathy associated virus. or LAV) grew in T4 cells but not in related cells called T8; that finding was made by David Klatzmann and Jean-Claude Gluckman of the Salpetriere Hospital in collaboration with the Pasteur group. It was shown that the virus could kill T4 cells or inhibit their growth. Electron micrographs of the new virus were different from those of HTLV-I and resembled those of a retrovirus of horses. A viral protein called P25 (or P24) that is not present in HTLV-I was identified. In collaboration with virologists from the Claude Bernard Hospital a blood test for lAY antibodies was formulated. Several examples of lAY or lAV-like viruses were isolated from homosexual men, hemophiliacs and central Africans. Early results of applying the blood test were suggestive but not fully conclusive. lAV antibodies were found in a large fraction of lymphadenopathy patients but in only a minority of AIDS patients. Yet the proportion increased as the sensitivity of the test improved. By October, 1983, it had reached 40 percent. At that point one of us (Montagnier) was convinced lAV was the best candidate for the cause of AIDS.

To the other one of us the evidence did not seem so clear. For one thing, results had been obtained (by Gallo and Essex) indicating that some AIDS patients are infected with HTLV-I or a variant of that virus. It is now known that those results stemmed partly from the fact that among people infected with HIV are some who are also infected with the HTLV's. Moreover, only a minority albeit a substantial one-of AIDS patients had shown serological evidence of lAY infection. In addition, when it was first isolated, lAY could not be grown in large amounts in continuous cell lines. Without large quantities of virus it was difficult to prepare specific lAY reagents that could be used to show that all people with AIDS or pre-AIDS were infected by the same type of virus.

Therefore on the American side much effort was concentrated on growing the pathogen from the blood of AIDS patients in mass, continuous culture. By the end of 1983 that task had been accomplished by the Gallo team: several cell lines had been identified that could support the growth of the new agent. The first reagents for specifically typing this virus were rapidly made. Employing those reagents, it was shown that 48 isolates obtained beginning in early 1983 from AIDS patients and people in risk groups were all the same type of virus, which was called HTLV-III on the American side. A blood test was formulated and used to show that HTLV-III was present in almost all people with AIDS, in a variable proportion of people at risk of the disease (including people who had received blood contaminated by the virus but had no other risk factors) and in no healthy heterosexuals. The cause of AIDS had been conclusively established.

These results confirmed and extended the ones from France. lAV and HTLV-III were soon shown to be the same virus. Before long an international commission had changed its name to HIV, to eliminate confusion caused by two names for the same entity and to acknowledge that the virus does indeed cause AIDS. Thus contributions from our laboratories in roughly equal proportions-had demonstrated that the cause of AIDS is a new human retrovirus.

That HIV is the cause of AIDS is by now firmly established. The evidence for causation includes the fact that HIV is a new pathogen, fulfilling the original postulate of "new disease, new agent." In addition, although the original tests found evidence of HIV infection in only a fraction of people with AIDS, newer and more sensitive methods make it possible to find such evidence in almost every individual with AIDS or pre-AIDS. Studies of blood-transfusion recipients indicate that people exposed to HIV who have no other risk factors develop AIDS. The epidemiological evidence shows that in every country studied so far AIDS has appeared only after HIV. What is more, HIV infects and kills the very T4 cells that are depleted in AIDS. Although the causative role of HIV in AIDS has been questioned, to us it seems clear that the cause of AIDS is as well established as that of any other human disease.

Soon after the causation was established, a series of findings began to fill in the scientific picture of HIV. In a remarkably short time the genetic material of the virus was cloned and sequenced (in our laboratories and several others). The genetic complexity of HIV began to emerge when a gene called TAT was discovered by William A Haseltine of the Dana-Farber Cancer Institute, Flossie Wong-Staal of the National Cancer Institute and their collaborators. Such complexity is significant because it underlies the capacity of HIV to remain latent for a long period, . then undergo a burst of replication, a pattern that may hold the key to the pathology of AIDS.

There were other significant early findings. One of us (Gallo), with his colleagues Mikulas Popovic and Suzanne Gartner, showed that HIV could infect not only the T4 cell but also another type of white blood cell, the macrophage. The same one of us, working with his colleagues Beatrice H. Hahn, George M. Shaw and Wong-Staal, found HIV in brain tissues. It seems possible that the macrophage, which can cross the blood-brain barrier, may bring virus into the brain, explaining the central-nervous-system pathology seen in many AIDS patients.

How the virus infects both T4 cells and macro phages became clear when Robin A Weiss of the Chester Beatty Laboratories and, independently, Klatzmann and the Pasteur group showed that HN enters its target cells by interacting with the molecule called CD4. CD4 has a significant role in the immune function of T4 lymphocytes and also serves as a marker for that group of cells. The early work by the British and French teams showed that HN infects cells by binding to CD4. Hence only cells bearing that marker can be infected. (Although CD4 is the marker for the T4 cells, it is also found in smaller numbers on some macrophages, allowing them to be infected.)

Several additional findings rounded out the early discoveries. The potential of the epidemic to spread beyond the original risk groups was shown when Robert R. Redfield and one of us (Gallo) demonstrated that HIV can be transmitted during heterosexual intercourse. Members of the Gallo team also showed that the genetic makeup of the virus is highly variable from strain to strain, a fact that may complicate the attempt to formulate an AIDS vaccine.

After the rapid initial advance the pace slowed somewhat and began to approximate that of a more mature area of research. Yet the continuing work was not without surprises. In October, 1985, one of us (Montagnier) was engaged in analyzing blood samples brought to his laboratory by a visiting investigator from Portugal. Many of the samples were from people who had lived in Guinea-Bissau, a former Portuguese colony in West Africa. Among them were some people who had been diagnosed by Portuguese clinicians and investigators as having AIDS in spite of the fact that their blood showed no sign of HN infection.

One sample, in fact, was negative for HN using the most sophisticated techniques available at the time. Yet workers in the laboratory were able to isolate a virus from the patient's blood. DNA "probes" (short pieces of DNA from the HIV genome) were then prepared. If the new virus were closely related to the original AIDS agent, those probes would bind to its genetic material. As it turned out, there was little binding, and it became clear that the new isolate was not simply a strain of the original AIDS virus but a new virus designated HN-2. Soon a second example was isolated by workers at the Claude Bernard Hospital; many others followed.

In evolutionary terms HIV-2 is clearly related to HIV-1, the virus responsible for the main AIDS epidemic. The two viruses are similar in their overall structure and both can cause AIDS, although the pathogenic potential of HN-2 is not as well established as that of the first AIDS virus. HN-2 is found mainly in West Africa, whereas HN-1 is concentrated in central Africa and other regions of the world. The finding of HIV-2 suggests that other undiscovered HIVs may exist, filling out a spectrum of related pathogens.

The isolation of HN-2 immediately raises the question of the evolutionary origins of these viruses. Although the answer to that question has not been found, some hints have been provided by the discovery in other primate species of related viruses called simian immunodeficiency viruses (SN's). The first such virus, found in the macaque monkey, is designated SN macaque. Isolated and characterized by Ronald C. Desrosiers and his co-workers at the New England Regional Primate Research Center in collaboration with Essex and his colleague Phyllis Kanki, SN macaque has been shown to be closely related to HIV-2, raising the possibility that HIV-2 may have come into human beings relatively recently from another primate species.

No such close simian relative has been found for HN-1 (although the right group of primates may not yet have been studied in sufficient detail). Hence the origin of HN-1 remains more mysterious than the origin of its relative HN-2. It is likely, however, that HN-1 has been in human beings for some time. One of us (Gallo), with Temin, has used the divergence among HN strains and the virus's probable rate of mutation to estimate how long the virus has infected people. It was tentatively concluded that HN has infected human beings for more than 20 years but less than 100, an estimate compatible with those by other workers and with our knowledge of the epidemic.

Where was HN hiding all those years, and why are we only now experiencing an epidemic? Both of us think the answer is that the virus has been present in small, isolated groups in central Africa or elsewhere for many years. In such groups the spread of HN might have been quite limited and the groups themselves may have had little contact with the outside world. As a result the virus could have been contained for decades.

That pattern may have been altered when the way of life in central Africa began to change. People migrating from remote areas to urban centers no doubt brought HN with them. Sexual mores in the city were different from what they had been in the village, and blood transfusions were commoner. Consequently HN may have spread freely. Once a pool of infected people had been established, transport networks and the generalized exchange of blood products would have carried it to every corner of the world. What had been remote and rare became global and common.

What weapons are available against this scourge? Perhaps the best weapon is knowledge. One key form of knowledge is a deeper understanding of HN, its life cycle and the mechanisms by which it causes disease. Although HN kills T4 cells directly, it has become clear that the direct killing of those cells is not sufficient to explain the depletion seen in AIDS. Indirect mechanisms must also be at work. What are they?

Many possibilities have been suggested. Infection by HIV can cause infected and uninfected cells to fuse into giant cells called syncytia, which are not functional. Autoimmune responses, in which the immune system attacks the body's own tissues, may also be at work. What is more, HIV infected cells may send out protein signals that weaken or destroy other cells of the immune system. In addition HN is fragile, and as the virus particle leaves its host cell, a molecule called gp120 frequently falls off the virus's outer coat. As Dani P. Bolognesi of the Duke University Medical Center and his co-workers have shown, gp120 can bind to the CD4 molecules of uninfected cells. When that complex is recognized by the immune system, cells thus marked may be destroyed.

That list does not exhaust the possibilities. One of us (Montagnier) is exploring the possibility that the binding of the virus to its target cells triggers the release of enzymes called proteases. Proteases digest proteins, and if they were released in abnormal quantities, they might weaken white blood cells and shorten their lives. The various proposed mechanisms are not exclusive, and several may operate at once. Yet one is probably central, and some of the most significant work on AIDS is that of distinguishing the central mechanism from the peripheral ones that accompany it.

Although it is clear that a large enough dose of the right strain of HN can cause AIDS on its own, cofactors can clearly influence the progression of the disease. People whose immune systems are weakened before HN infection may progress toward AIDS more quickly than others; stimulation of the immune system in response to later infections may also hasten disease progression.

Interaction with other pathogens may also increase the likelihood that AIDS will develop. Specifically, a herpes virus called human B-cell lymphotropic virus (HBLV) or human herpes virus 6 (HHV-6) that was discovered in the laboratory of one of us (Gallo) can interact with HN in a way that may increase the severity of HN infection. Ordinarily HHV-6 is easily controlled by the immune system. In a person whose immune system is impaired by HN, however, HHV-6 may replicate more freely, becoming a threat to health. In addition, although one of the main hosts of HHV-6 is a white blood cell called the B cell, the virus , can also infect T4 lymphocytes. If the T cell is simultaneously infected by HN, HHV-6 can activate the latent AIDS virus, further impairing the immune system and worsening the cycle.

Clearly, in spite of rapid progress there are many gaps in our understanding of HN and AIDS. Should we panic? The answer is no, for several reasons. The most obvious is that panic does no good. The second reason is that it now seems unlikely HIV infection will spread as rapidly outside the original high-risk groups in the industrial countries as it has within them. A third reason is that this disease is not beyond the curative power of science. Although current knowledge is imperfect, it is sufficient to provide confidence that effective therapies and a vaccine will be developed.

The possibilities for therapy are particularly impressive. In the first phase of the search for AIDS therapies it was necessary to exploit any drug that seemed to provide even a remote chance of combating HIV infection. A variety of compounds formulated for other purposes were taken off the shelf and tested. Most were of little value, but one (AZT), originally formulated as an anticancer drug, turned out to be the first effective anti-AIDS agent. More recently, an experimental regimen in which AZT is alternated with the related compound known as dideoxycytidine offers even greater promise.

Bringing AZT into clinical use was a significant accomplishment, because it gave hope that AIDS would not remain incurable forever. As a form of therapy, however, AZT is not perfect and will probably be supplanted by less toxic agents formulated on the basis of what is known about the HIV life cycle. One promising agent is CD4, the molecule that serves as the viral receptor. Early tests show that soluble CD4 can bind to the virus and prevent it from infecting new cells. Many other drugs are in trials; one of them, perhaps combined with compounds that bolster the immune system, may provide therapy for HIV infection.

In assessing the progress that has been made toward achieving fully effective AIDS therapy, it must be kept in mind that this work has two facets. In addition to combating a complex and evasive pathogen, it must pioneer entirely new areas of medicine. The reason is that there are few effective treatments for viral diseases-and almost none for retroviruses. There are various reasons for this, among them the fact that viruses (unlike bacteria, for which effective therapies exist) always appropriate the biosynthetic apparatus of the host cell. As a result drugs effective against viruses tend to damage mammalian cells. Yet we are confident that the dual goals of pioneering science and clinical effectiveness will be met.

What is true of therapy is also true of vaccines: an AIDS vaccine will be a pioneering scientific achievement. Since the HIV genome has the capacity to integrate into the chromosomes of the host cell, little serious consideration has been given to using preparations containing the whole virus as a vaccine. An AIDS vaccine must consist of subunits, or parts, of the virus in the right combination. Yet experience with subunit vaccines is slight. Indeed, so far only a few subunit vaccines have proved practical. Much work is under way to find the combination of HIV subunits that will yield the greatest protective response. As in the case of therapy, we believe there will be a practical vaccine against HIV.

Perhaps an even more persuasive reason for hope is that even without a vaccine or a cure, what is already known could bring the epidemic under control. The blood supply has already been largely secured by the presence of a blood test. Moreover, the modes of transmission of HIV-blood, sexual intercourse and from mother to child-are firmly established. Hence any individual can drastically reduce his or her risk of infection. If such knowledge were applied everywhere, there would be a sharp leveling off in the spread of HIV infection, as there has been in some groups in the developed world. The lesson here is that there is a need for education about HIV infection--in clear, explicit language and as early as possible.

Yet there are parts of the epidemic where education alone is not sufficient, and it is in those areas that humanity will be tested. Users of intra· venous drugs, for example, are notoriously resistant to educational campaigns alone. It seems clear that the effort to control AIDS must be aimed in part at eradicating the conditions that give rise to drug addiction. Those conditions are in turn linked to social and economic patterns. Eliminating the disease may entail eliminating some of the social differentials that form the substratum of drug abuse.

It is also the case that in some areas of the developing world education alone will not stem the epidemic. Education is necessary, but it must be accompanied by other measures. In central Africa--the part of the world most beleaguered by AIDs--there are few facilities for blood testing and few technicians trained to perform tests. Furthermore, the blood tests used in the U.S. and Western Europe are too expensive to be helpful. As a result the virus is still being spread by contaminated blood, long after that form of transmission has been practically eliminated in the industrial countries.

To help change this situation the World AIDS Foundation has made improving the situation in central Africa its highest priority. The foundation (along with its parent, the Franco-American AIDS Foundation) was formed as part of the agreement that resolved a lawsuit between France and the U.S. over the AIDS blood test. The parent foundation receives 80 percent of the royalties from the French and American blood tests; the World AIDS Foundation in turn receives 25 percent of that. Much thought has been given to how to allocate the funds, and the first project (carried out in conjunction with the World Health Organization) will be realized in several African countries. It will include training technicians to perform blood tests, establishing one HIV-free blood center and increasing public education about HIV transmission.

Efforts such as this one, coupling public and private funds and energies, will be essential to stopping AIDS. As we stated above, both of us are certain that science will ultimately find a cure and a vaccine for AIDS. But not tomorrow. The AIDS virus (and other human retroviruses) will be with us for a long time. During that time no intelligent person can expect the necessary solutions to come solely from authorities such as scientists, governments or corporations. All of us must accept responsibilities: to learn how HIV is spread, to reduce risky behavior, to raise our voices against acceptance of the drug culture and to avoid stigmatizing victims of the disease. If we can accept such responsibilities, the worst element of nightmare will have been removed from the AIDS epidemic.