JUNE marks 30 years since the first report of AIDS – a syndrome that has killed an estimated 25 million people worldwide. Yet this year’s anniversary is somewhat arbitrary: the virus responsible for AIDS has probably been circulating within human populations for 100 years. Why did it take so long to detect it?

In June 1981, doctors reported an unusual type of pneumonia in five previously healthy young homosexual men in Los Angeles. Two years later the cause of their immunodeficiency was identified: a retrovirus that targets white blood cells,subsequently named human immunodeficiency virus (HIV).

Similarities with the simian immunodeficiency virus (SIV) that infects chimpanzees in west-central Africa suggest the original source of the infection, which probably spread to people who hunted and ate the apes.

Thanks to two chance finds in medical samples collected 50 years ago in what is now the Democratic Republic of the Congo (DRC), we even know roughly when HIV arrived. In 1998, researchers found HIV in a blood sample collected in 1959. This was followed in 2008 by a second discovery of the virus in a sample collected from a woman’s lymph node in 1960. The two viruses were subtly different due to their independent evolutionary histories. Comparing their gene sequences established that they likely diverged from a single common ancestor between 1902 and 1921, suggesting HIV has been in human populations for at least that long. Gene sequences also reveal that HIV spread from Africa to Haiti – probably shortly after what is now the DRC gained independence from Belgium in 1960 – and arrived in the US around 1969.

For Paul Sharp at the University of Edinburgh, UK, it is obvious why HIV went undetected for 70 years. If infection follows an exponential curve, he says, there may have been just 4000 cases in west-central Africa in 1960. The researchers who found the two samples can count themselves “very lucky” to have done so, he says.

Michael Worobey at the University of Arizona in Tucson is one of those researchers. “Finding those specimens did involve luck – but also time, energy and perseverance,” he says. He thinks HIV evaded detection for other reasons. There is a delay of about 10 years between infection and onset of symptoms. And AIDS isn’t associated with a specific suite of symptoms. “HIV causes you to die from any number of other infections.” In an area like sub-Saharan Africa, where a number of fatal diseases are already rife, it is only with hindsight that some of those deaths can be attributed to HIV.

Worobey says that what is truly telling is not that HIV circulated unnoticed in Africa for 70 years, but that it went undetected in the US between 1969 and 1981. With an exponential rate of spread, there may have been about 100,000 infections by the time the first cases were reported in 1981. “It took 12 years and 100,000 cases in a developed country to detect HIV, so it’s not a mystery that it remained hidden for so long in sub-Saharan Africa,” he says.

“The past 30 years really is just the tip of the iceberg in the history of HIV.”