How did SSGT Johnson escape the flood but not Keyes?

Just got to the flood part in Campaign Evolved and the cutscenes do make the situation look more clear than Combat Evolved. You see Keyes and Johnson as the last men standing side by side.

So how did Johnson escape while Keyes get captured?

I know in the Mona Lisa the Flood doesn't infect personnel who might be important (so they can feed to the proto-gravemind).

I also heard that the whole Johnson having higher resistance and "Johnson has inactive flood DNA in his body" got retconned but I haven't really confirmed that.

So how did Johnson escape the flood but not Keyes?