This treatment works because HIV is a retrovirus. Retroviruses insert their own DNA into the human genome and force us to use their genes with our cellular machinery to create more viruses.

CRISPR is a gene editing technology that can cut DNA at precise locations. This means we can cut the HIV DNA out of the cell's genome and reverse the infection.

This will not work for every virus though because not every virus works by inserting their genome into ours. So, even if it were not an extremely risky procedure, we could never use this tech to cure a virus such as influenza.