LOS ALAMOS, N.M. (AP) – Studies by Los Alamos National Laboratory suggest HIV spreads more quickly than previously thought, making the infection tougher to fight.

Previous estimates – looking at a cell at a single point in time – suggested that 100 to 200 viruses might be made in each infected cell, researcher Alan Perelson said. That estimate later was raised to 1,000 to 2,000, he said.

But lab researchers studying individual cells of SIV – the simian version of HIV – found a much faster rate. SIV and HIV act similarly, so it’s reasonable to think HIV could behave the same way, Perelson said.

“When we looked at a cell over its life span, we found each cell was making approximately 50,000 viruses – and it looks like that’s the minimum,” he said.

Researchers previously looked only at how much virus was produced from a cell at a specific time. But in studying how much SIV was produced in cells of rhesus monkeys, scientists could determine how much virus was produced from a cell over its life.

“The other method is like looking at a General Motors manufacturing plant and seeing 100 cars on the line and saying, ‘Oh, that plant makes 100 cars,”” Perelson said. “But we know it makes a lot more than that over time.”

The SIV strain engineered by scientists could infect one cell and produce offspring, but the offspring were defective and couldn’t infect any other cells. Afterward, researchers counted the virus reproduced from the one cell.

It’s difficult to do a similar test on humans because the subject has to be dead before scientists can count how much the virus has reproduced.

Still, Perelson said, the conclusions are similar.

“Overall, though, this tells us the infection is a lot tougher to combat,” he said. “Early in the infection, sharing needles, blood, if a small number of cells are transferred, the disease has a larger chance of spreading through the body quickly.”

An HIV expert, lab fellow Bette Korber, said the results are helpful as a way to study HIV. She said, however, the knowledge cannot be applied directly in creating a vaccine.

“This lets us know more what we’re up against,” she said. “Maybe it tells us something about the efficacy of a vaccine. Maybe you can’t protect against infection, but you could try to find a way to stop the progression of HIV.”

Perelson said the study is another step toward finding a cure.

“You have to know your enemy before you can fight it,” he said.