Board Thread:Warhammer 40k General Discussion/@comment-4851119-20140719123726/@comment-6078851-20150611144518

since the life eater is organic tyranids can collect samples of 'used' life eater virus (as in collecting the goo of its victims that have been exposed to it). then they could have access to the inert (used) virus and can start creating life eater resistant bioforms. and they could also potentially create their own life eater which could target everything except tyranids, after which all that would be left of opposition would be sludge ready to slurp :)

or, if used in space (if it actually can be used in space), tyranids bioships can simply spit out 'interceptor' bioforms in the millions to prevent life eater warheads from impacting hive ships.

or, tyranids could start creating extremely long-range warships to destroy any ships carrying such munitions before they're launched, and with undetectable ships tyranids wouldn't really have a problem scouting, and detecting, vessels armed with life eater munitions.

if it can't be used in space then tyranids just need to make sure that all life eater-capable imperial ships are utterly destroyed before they start chowing down on a world.

so that's a couple of countermeasures off the top of my head (and my intellect is literally nothing compared to the hive mind, so god knows what else it could come up with :P). the only way i think the life eater would work to entirely wipe out tyranids is that if it works in space (which i'm fairly sure it doesn't), and if the entire race is attacked at the same time (to within a day or 2), which they are far, far too numerous to do (plus most tyranids aren't yet within the boundries of the galaxy).