The number of Americans infected with the HIV virus each year is much higher than current government estimates, US health officials have said.
The Centers for Disease Control and Prevention (CDC) said 56,000 people had become infected with the virus that causes Aids in 2006.
That is substantially more than the earlier annual estimate of 40,000.
However, the CDC said the rise was due to improved detection methods, rather than an increase in infections.
For the first time, new blood tests can tell how recently an HIV infection occurred, allowing researchers to pinpoint the year it happened.
The increase is also thought to be due to new statistical methods.
The CDC added that the annual number of new infections was never as low as 40,000, and that it has been roughly stable since the late 1990s.
"This is the most reliable estimate we've had since the beginning of the epidemic," said CDC director Julie Gerberding.
In a statement, the CDC went on to say that the new estimates highlight the need for expanded HIV prevention services and "should serve as a wake-up call that the US HIV/Aids epidemic is far from over".
The executive director of the National Alliance of State and Territorial Aids Directors, Julie Scofield, told the Associated Press news agency: "This is the biggest news for public health and HIV/Aids that we've had in a while."
The revised estimated and the methodology behind it are due to be presented at the opening day of an international Aids conference in Mexico City on Sunday.
(BBC)
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