AIDS first came to public notice in 1981 when US doctors noted an unusual cluster of deaths among young homosexuals in California and New York. However, it appears that the disease has been around for more than initially believed. A study published in the journal Nature concluded that genetic analysis estimates that HIV (the virus that causes AIDS b destroying immune cells) arose between 1884 and 1924, with a more focused estimate at 1908. The previous estimation was around 1930.

The new finding “means the virus was circulating under our radar even longer than we knew,” Michael Worobey, of the Department of Ecology and Evolutionary Biology at the University of Arizona in Tucson, said.

For the study, Dr. Worobey and colleagues analyzed two of the earliest samples of the virus ever found: a 1960 sample of HIV gene fragments from a wax-embedded lymph-node tissue biopsy from a woman with HIV-1 group M genetic sequence and a 1959 blood sample from a man, both from Kinshasa, in the Democratic Republic of the Congo. These are the oldest samples of HIV. The next oldest sequences of HIV are from the late 1970s and 1980s, when scientists already knew about the existence of AIDS.

HIV-1 is the most common form if the virus known to have originated in chimpanzees because of close genetic similarities to a simian virus. Previous studies have shown that HIV spread from these animals to humans in southeastern Cameroon.

Comparing the two samples of HIV, the researchers traced their common ancestor to between 1884 and 1924, some 50 years earlier than previously known.

“Now, for the first time, we have been able to compare two relatively ancient HIV strains. That helped us to calibrate how quickly the virus evolved and make some really robust inferences about when it crossed into humans, how the epidemic grew from that time, and what factors allowed the virus to enter and become a successful human pathogen,” Dr. Worobey said.

He further added that the key to the success of the virus was possibly the development of cities such as Kinshasa (known as Leopoldville) in the early 1900s. Such cities provided more chances for infected people to pass the virus to others, one of the reasons for the quick spreading of the virus. The virus was transmitted through sex at first, but then was taken further afield through commerce.

Dr. Worobey believes the findings will enable scientists to find the source of HIV on the one hand, but on the other hand, they will help finding a cure or better ways of prevention.

Since 1981, AIDS has killed at least 25 million people, and 33 million others are living with the disease or HIV. A report by the US Centers for Disease Control and Prevention released last month concluded that homosexuality seems to be the main cause of new HIV infections. And when it comes to young black gay men, the risk of infection is seven times higher than in the case of the whites.