Scientists discovered a new strain of HIV for the first time in 19 years.
The guidelines on how to classify group M HIV were established in 2000 and this is the first new strain of the virus since these guidelines were implemented.
Health company Abbott published the findings of its virus tests in the Journal of Acquired Immune Deficiency Syndromes.
The new HIV-1 group M strain, subtype L, could only be officially identified after confirmation of three cases.
The first two cases were discovered in the 1980s and 1990s in the Democratic Republic of Congo.
A third sample was collected in 2001, but tests at the time failed to confirm due to sample size and technology at the time.
Now that the new strain has been identified, HIV treatments will not be affected, but the tests will be modified to provide strain detection.
The group M HIV subtype is considered the leading cause of the global HIV pandemic.
Carole McArthur, professor in the oral and craniofacial sciences departments at the University of Missouri, Kansas City, was one of the authors of the study.
She said: ‘In an increasingly connected world, we can no longer think of viruses contained in one place.
"This finding reminds us that to end the HIV pandemic, we must keep thinking about this ever-changing virus and use the latest advances in technology and resources to monitor its evolution."