It was only a decade ago. The medical community was feeling quite secure. It had stamped out smallpox, put a hold on polio, held tuberculosis at bay. Vaccines and antibiotics had saved us from bacterial and viral infections; wide-scale epidemics were now a topic for history books.
But the picture changed drastically in 1981 when doctors first diagnosed a patient as having AIDS. A new disease had appeared on the horizon, an illness caused by a contagious agent now known as a retrovirus. The new epidemic took many off guard.
Aside from AIDS, small-scale epidemics caused by related retroviruses emerged in isolated regions around the world. These cousins of the AIDS virus carry the blame for some forms of cancer and neurological disorders.
Thus far scientists have found no cures for any human retroviral diseases. They have concentrated their efforts on studying AIDS, for which few treatments are available.
But the knowledge they gather in the search for a cure for AIDS is likely to provide a strong foundation for understanding and treating other retroviral disorders.
The unique viruses contain the preface "retro" because they act in a reverse direction. In all known organisms, the genetic material DNA is converted to RNA and then to proteins. In these "backward" entities, however, the hereditary information is carried by RNA, which is translated backward to DNA. The resulting DNA then infects the host cell, incorporating itself into the host's genetic material where it can later replicate.
In 1910, Peyton Rous identified the first retrovirus and found that it could cause tumors in chickens. During the next 70 years, scientists continued to study these pathogens in other farmyard creatures such as mice, cows and cats. Retroviruses, they learned, were clearly linked to cancer. If the pathogens could induce malignancies in animals, scientists reasoned, why not also in humans?
But by the late '70s, no one had been able to isolate a retrovirus from humans. Many researchers were beginning to doubt that any type of virus was involved in cancer. The virus cancer program at the National Institutes of Health had closed down. Convinced that retroviruses were unique to animals, many believed retroviral research should be scaled back.
However, in 1979, Dr. Robert Gallo of the National Cancer Institute proved the skeptics wrong when he identified the first human retrovirus, human T-cell lymphotrophic virus-1 (HTLV-1). Victims carrying the virus suffered from a rare cancer: adult T-cell leukemia (ATL). Since HTLV appeared to affect only a small segment of the population, the discovery didn't receive much attention in the press. Scientists identified the first cluster of patients on two southern Japanese islands: Kyushu and Shikoku. Later, more cases surfaced in the Caribbean, parts of South America and Africa.
The disease remained in small, circumscribed regions for a specific reason. Unlike common cold viruses, which can be sneezed from carrier to carrier, HTLV followed a more selective route of infection: by blood transfusions, sexual contact or from a pregnant woman to her growing fetus. Once transmitted, the pathogen could hibernate in the victim's cells for as long as 40 years before symptoms appeared. Then the cancer began. Specialized blood cells, called T4 lymphocytes, would proliferate wildly. Patients were overwhelmed by fatigue and deteriorated quickly, dying within three to four months.
At the time, the discovery of HTLV seemed inconsequential to the public. Adult T-cell leukemia touched the lives of few people in the United States. Two years later, however, the rise of AIDS changed public indifference. The fatal syndrome contained many similarities to HTLV-linked leukemia.
The parallels to ATL were striking. AIDS infection, as with ATL, often occurred years before symptoms manifested themselves. The disease was transmitted sexually, through blood or from a mother to her unborn child. The syndrome also produced abnormalities in the same cell population, T4 lymphocytes. However, in AIDS, the infectious agent destroyed massive quantities of blood cells. In contrast, with ATL, those same T4 lymphocytes exploded in a cancerous growth.
"It just seemed too coincidental that all those things fit," Max Essex of the Harvard School of Public Health said. A related virus must be involved in AIDS, he and many of his fellow scientists believed. With the discovery of HTLV, the tools had been developed to study human retroviruses. Now scientists could use these tools to quickly track the infectious agent responsible for AIDS. As anticipated, a retrovirus, known today as human immunodeficiency virus (HIV), turned up as the cause of AIDS.