A new study by the US Centres for Disease Control and Prevention (CDC) indicate that almost one million Americans are now living with HIV infections and about half of this number are undiagnosed or untreated, reports a US gay website.

The findings presented Monday in Seattle at the 9th Conference on Retroviruses and Opportunistic Infections revealed that new infections have remained steady at around 40,000 per year and between 850,000 and 950,000 Americans are living with HIV and AIDS.

The centre has noted that the number of people living with AIDS is increasing as death rates from AIDS have fallen as a result of the widespread use of "highly active antiretroviral therapies" which are prolonging the lives of those with HIV/AIDS, said lead CDC researcher Dr Patricia Fleming.

Using random testing and statistical analyses methods, the CDC estimated that about a quarter of those infected do not know they have the disease. Researchers added that this group is at the highest risk for spreading the illness.

Dr Harold Jaffe, director of the CDC's National Centre for HIV, Sexually Transmitted Diseases and Tuberculosis Prevention, said: "The take-home message is that there are more people living with HIV than ever before, there are too many who are diagnosed too late, and there are an unacceptable number not receiving treatment and services."