Sentience

"They and all additional sentient life within three radii of the galactic center, died as planned."

- 343 Guilty Spark

A sentient is a being with the ability to think intelligently, commonly determined by the being's ability to speak, manipulate tools, and reason. Other lifeforms lacking such capabilities are classified as non-sentient, though some classification systems have a middle ground, semi-sentient, for species that had very primitive capabilities to manipulate tools, use language, and/or solve problems.

In Relation to the Flood
The flood only consumed an organism that was sentient and also had a sufficient biomass to support the infection process. for the most part all species in the covenant are sentient but some lack biomass needed to be infected, hunters cannot be infected because each sentry are made from a buildup of many eels that lack a spinal cord.