Briefing: "A nearby GDI base is conducting further ion research. In doing so, they use a lot of chemicals, located in their bio centers. Destroy all the bio centers, and "Contaminate" all units and civilians. Leave all other structures intact so that it looks like an accident."
Walkthrough: Fourth Nod mission of The Covert Operations datadisk for C&C Tiberian Dawn. This is the tactical one, and not so hard. You can speedrun this right to the GDI base. I did take more safe approach and did "proper" run through the map.