There are other gene editing systems out there, such as zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENS.) ZFNs are nucleases fused to a zinc finger DNA binding domain. You can engineer ZFNs to target a specific nucleic acid, just as CRISPR-Cas9 does. However, CRISPR-Cas9 systems are revered for their simplicity, essentially just needing the DNA sequence you want to target. TALENs functions like ZFNs, a nuclease bound to a TAL (transcription activator-like effector), which recognizes a specific DNA site. There are other gene editing tools, you can see that among Cas9, ZFNs, and TALENs, the structure/function of the tool is conserved.

Cancer cells and cells infected with HIV have shown to respond promisingly to CRISPR-Cas9 treatment. Ironically, CRISPR systems originated in prokaryotic species, as a defense against mobile genetic elements (MGEs), like viruses or bacteriophages. But CRISPR-Cas9 is being studied as a defense against viral infections, targeting the viral nucleic acids at different stages.