The mutagen is
very often green, glowing ooze. Possibly radioactive waste or similar.
Unlike the real-world equivalents, this is for those that cause rapid and noticeable changes in the infected creature, often transforming them completely in short order.
Cases only heard about or seen only in cinematics should NOT count, though they can still be added if a note is included about the limited exposure.
Sanguine # 2009-07-15 22:37:01