"Three benign particles. In combination, they form a quasi-virus. A programmable virus. Deadly to Yeerks. There is one problem. One of the components is subject to ... to simplify, it has a volatility that could cause it to mutate in a Yeerk with a human host."
Estrid-Corill-Darrath to Aximili-Esgarrouth-Isthill[src]

A prion virus was developed by Estrid-Corill-Darrath at the University of Advanced Scientific Theory on the Andalite homeworld, that was designed to cripple Yeerks while also causing a negative mutation in humans. It was nearly used by Arbat-Elivat-Estoni on the Yeerk pool in the Animorphs' hometown, although the virus vial was destroyed by Estrid.[1]

Andalite technology Dome Ship | Escafil Device | Hirac delest | Mirrorwave call | Z-space transponder | Morphing | Mag-Hover Truck | Quantum Virus | Shredder | Gleet Bio-Filter | Prion Virus
Yeerk Technology Anti-morphing ray | Dracon beam | Gleet Bio-Filter | Kandrona | Kandrona Rays | Nova-class Empire Ship | Pool ship | Blade ship | Mag-Lev Train | Hunter Robot | Yeerk Pool | Bug fighter
Ketran Technology Explorer | Time Matrix | Mapping Crystal Quadrant 3 | Uninet | Adge
Other Dropshaft | Ixcila (Arn) | Pemalite crystal | Pemalite ship | Skrit Na Saucer | Hawjabran Colony Ship | Hawjabran Freighter