The rate of HIV infection among injecting drug users appears to be rising, researchers say.
The report, published in the British medical journal The Lancet, says 3m self-injecting drug users worldwide could now be HIV-positive.
In nine countries, more that 40% of drug users were infected.
The authors are concerned about the lack of data from Africa and say the risk factors that have helped spread HIV in this way exist on the continent.
They said there was a "pressing need" to address the problem.
The scientists behind this study, frmo the University of New South Wales in Australia, carried out a wide-ranging review of published data.
They concluded that both the numbers of injecting drug users and the prevalence of HIV infection among them are on the increase.
The virus is spread mainly by the use of shared needles.
In some countries in South East Asia, Latin America and Eastern Europe the rates of infection among injecting users are above 40%. In Estonia it is more than 72%.
But some countries have maintained very low rates of infection, such as the UK - where the rate is 2.3% - New Zealand and Australia where only 1.5% of injecting drug users are HIV-positive.
Researchers say that this was due to the swift introduction of needle exchange programmes in the 1980s.
The report says that there is a clear mandate to invest in HIV prevention programmes such as needle exchanges and drug substitution treatments.
There is also a clear need for education to help prevent the spread of infection in countries where injecting drug use is common but where the virus has not yet become widespread among users.
"The high prevalence of HIV among many populations of injecting drug users represents a substantial global health challenge," the authors say.
Michael Carter, of the HIV information service NAM, said the report provided "striking evidence" to support the provision of needle exchange programmes.
He said: "The UK has historically low rates of HIV amongst injecting drug users because we put health promotion first.
"Compare this to the HIV prevalence in countries which have resisted the provision of needle exchanges in the misguided belief that they 'encourage' drug use."
Mr Carter added there was good evidence that IDUs can do very well on anti-HIV treatment - but often stigma and discrimination meant that they were denied potentially life-saving drugs.
Ailsa Spindler, of the charity Terrence Higgins Trust Scotland, said: "There is a long way to go globally in tackling IDU, its causes and its effects on health, including HIV transmission."
She said health among IDUs was "relatively good" in the UK, but the number of new HIV diagnoses among the group had increased gradually over the past five years.
DRUG USER HIV BLACKSPOTS
Countries where over 40% of injecting drug users are thought to be HIV positive:
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