causes of zombification

Rewatching the remake of The Crazies today, thinking about how many other zombie/infection movies start with the water.
Found another post listing a few of those watery films here: ALL THINGS HORROR.
So I went on to think about examples of other virus/unexplained source of a zombie-mutating horror.

• 28 Days Later (2002)/28 Weeks Later (2007)
• [Rec] (2007)/Quarantine (2008)
• Dawn of the Dead (1978)
• Day of the Dead (1985)
• Demons 2 (1986) - yeah well, demons released from the TV count as zombie-like behavior.
• Dead Alive (1992) - the bite of the Sumatran rat-monkey (pic from gutzngory.com)
• Night of the Comet (1984) - was, of course, the gases of the comet passing through earth's atmosphere.
• Night of the Creeps (1986) - aliens (pic from forum.dead-donkey.com
• Night of the Living Dead (1968) - unexplained
• It Came From Outer Space (1953) - aliens
• Invasion of the Body Snatchers (1956, 1978, 2007) - aliens
• Return of the Living Dead (1985) - government cover up of unexplained fluid
• SLiTHER (2006) - aliens (pic from moviestudio.wordpress.com)

and of course, Stephen King's Dreamcatcher (2003), which uses all of the above, essentially.

If you were to make a zombie (or zombie-like) film, which cause would you choose? or would you leave it ambiguous?