The number of Americans living with HIV is growing every year

"We estimate that roughly half of all people living with HIV either don't know that they are infected, or they are not in care, or both," said Dr Harold Jaffe, an acting director of the National Center for HIV at the CDC.

The findings raise concerns that many people may be spreading the disease unintentionally, through unprotected sex or shared needles.

More people are living with HIV than ever before.

According to CDC epidemiologist Dr Pat Fleming, 50,000 more people have HIV or Aids than was the case two years ago.

But this is not necessarily a negative trend, because new treatments - although not providing a cure - are allowing those with the virus to live much longer before contracting full-blown Aids.

"Prevalence is increasing. That's a good thing because it means that people are surviving longer," said Dr Fleming.

But the new CDC data suggests that not enough people are accessing the drugs on offer.

"We've got to do a better job at promoting testing by health care providers, expand our information campaign, encourage people to recognise their risk behaviour and seek testing," said Dr Fleming.