Usually, whichever strain of the virus infected a survivor is what the victim will turn into. However, the NIGHT Virus is prone to mutation. During the Turning Phase, when the virus is in its most active state, it is possible for the strain to mutate in order to create what is known as a "cousin breed." Cousin breeds usually have similar traits or behavior to their parent strain. For example, the Phantom is a cousin to the Mimic. Both possess vocalization capabilities and echo sounds they hear. However, the Phantom has mutated and evolved to become more intelligent and only mimic select phrases.

The Binge is unique in that its strain will battle and overpower an existing infection so long as the host has not completed the Turning Phase. This has led to very rare instances of mixed breeds such as the Coalesce, which is the daughter breed of a Binge and a Spittoon's strains. Mixed breeds are exceedingly rare, however, and occur much less frequently than typical mutations.

Even more unique are conditional strains. Snares and Pyres seemingly occur circumstantially, though it is unknown how, as they are incapable of spreading the virus. Mer, on the other hand, are able to both spread the virus and acquire more of its kind circumstantially. Survivors do not know how it is possible for the virus to spread outside of fluid exchange or blood exposure.