A new strain of the human immunodeficiency virus (HIV) has been discovered by researchers in the United States, the first in almost two decades. The discovery comes after guidelines for what counts as a new subtype of the HIV pathogen were revised in 2000.

There must be at least three independently discovered samples containing a virus subtype before it can be classified as new. Scientists are also required to sequence the entire genome of all three, which can take months or years to complete.