PARIS -- They are tiny, stealthy and oh so patient, able to wait years before making patients sick and then slowly extinguish their lives.
They are two viruses that cause AIDS and cervical cancer.
Today, they are public enemies, branded over the deaths of tens of millions of people.
Yet just a quarter-century ago, these killers led an untroubled secret life.
On Monday, three scientists were awarded the 2008 Nobel Prize for Medicine for unmasking the human papilloma virus (HPV), the leading cause of cervical cancer, and the human immunodeficiency virus (HIV) that causes AIDS.
German scientist Harald zur Hausen was awarded half of the prize for finding that HPV causes cervical cancer, often dubbed a "silent killer" as the disease is often diagnosed far too late for treatment.
The other half is shared by France's Luc Montagnier and Francoise Barre-Sinoussi, who in 1983 linked a new virus, later named HIV, to acquired immune deficiency syndrome (AIDS).
That breakthrough was amplified by Robert Gallo of the United States, who confirmed HIV as the cause of AIDS.
Untold lives have been saved by this work, unfolding in two separate arenas of virology united by a common thread: the drive to prevail against conventional thinking.
Thanks to zur Hausen's persistent belief that viruses can cause cancer, there are two highly effective vaccines as well as simple tests for HPV.
"Scientific discoveries, such as this one, make a true difference when they can save lives," Kathleen Irwin of the World Health Organization (WHO) Initiative for Vaccine Research told AFP.
Of the hundred HPVs found so far, 15 place women at high risk of cervical cancer by transmission through sexual intercourse.
There were more than 260,000 deaths from this disease in 2005, making it the second leading cause of cancer in women worldwide, and the leading cause of cancer among women in developing countries.
"With HPV vaccines and screening tests, we hope to prevent most cervical cancer deaths in the near future," Irwin said.
The outlook is not nearly so promising for AIDS, which has killed at least 25 million people since 1981, when it was first recorded among a group of American homosexuals by alert epidemiologists.
Around 33 million people today have HIV, two-thirds of whom live in sub-Saharan Africa, a devastation that Barre-Sinoussi admitted Monday had been utterly unexpected back in 1983.
HIV has turned out to be a nightmare -- a mutant, slippery foe that can be transmitted through sexual intercourse, blood transfusions, shared use of contaminated syringes and from an infected mother to her unborn child.
There is no cure and every attempt to devise a vaccine has run into the ground. After repeated false dawns, many vaccine researchers today are deeply pessimistic.
But Kevin de Cock, the director of the HIV/AIDS Department at the WHO, said it was essential to realize how revolutionary the early breakthrough was and understand the benefits it brought.
"When I was a medical student in the 1970s, people thought infectious diseases were basically a thing of the past. What hubris!" he said.
"(The 2008 Nobel) is a testimony to the importance and complexity of infectious diseases, and especially to the field of virology. Both of these issues -- HPV and its link to cancer, HIV and its link to AIDS -- are enormous problems in global health."
By discovering HIV, the way was open to an antibody test for the virus in blood samples, he said. This protected blood banks transmitting from the pathogen in transfusions and informed infected individuals that they too could place others at risk.
By the mid-1990s, Big Pharma came up with antiretroviral drugs, which keep HIV at bay, transforming it from a death sentence to a manageable, if chronic, disease.
Compared with other novel diseases that have leapt out, this speed was unprecedented, the Nobel committee noted.
"Never before has science and medicine been so quick to discover, identify the origin and provide treatment for a new disease entity," it said.
De Cock said this astonishing early work and other research has yielded many spinoffs for other viral diseases such as hepatitis B and bird flu.
The benefits include tools for imaging viruses and cracking their genetic code and insights into how an agent is transmitted or holes up in an animal reservoir.
