CRISPR-Cas adaptive immune systems are a unique form of adaptive immunity found in prokaryotes wherein viral DNA or RNA sequences are stored on the host genome in the form of short (~30bp) "spacers". These spacers, stored at a repetitive CRISPR "array" can be used to reliably associate viruses with their hosts. This approach, though precise, will inevitably miss host-virus pairs where the host lacks a CRISPR array ( a majority of bacteria, especially certain groups), or where the diversity of host CRISPR spacers has been inadequately profiled. Nevetheless, this method can give highly confident host-virus pairs when a perfect match is found.