Viruses using CCR5 co-receptors (also known as R5 viruses/R5-tropic strains) are responsible for viral transmission and establishment of infection, while viruses using CXCR4 (also known as X4 viruses/X4-tropic strains), or both co-receptors (also R5X4 viruses/dual-tropic strains) have been identified at later time points during disease progression [38]. Here, CCR5 is linked to infection.