The advantage that a particular strain of an organism has over other strains in the same population. The fittest strain of HIV, e.g. a drug-resistant one in an environment in which drug is present, will replicate faster than less fit strains and become the dominant one.

A class of antiretroviral agent that binds to the gp41 envelope protein and blocks the structural changes necessary for the virus to fuse with the host CD4 cell. When the virus cannot penetrate the host cell membrane and infect the cell, HIV replication within that host cell is prevented.

Fusion is an integral step in the process whereby HIV enters cells. Researchers have found that, in addition to the primary receptor (the CD4 molecule), other cofactors, such as CCR5 and CXCR4, are needed in order for HIV to fuse with the membranes of the immune system cells.