Virus it is then. (:
The war starts out conventional (guns and various explosives) then biological weapons are used, which would, of course, be illegal. The army/government is telling everyone the 'virus' spreading in their country (what should we name it? perhaps some slang names as well?) is simply a mutation of an every day flu (sort of like the swine flu... but that genuinely was... this wouldn't be). When people start dying and quarantines are set up people start going crazy. Peaceful protests against the war turn into violent protests against the government/virus. Civil war breaks out within the attacked country(ies?) amongst those that aren't ill but it is slowly coming to a stop as the virus spreads.
... what would the country(ies) that this country is at war with be doing?
Ooh... or perhaps the government is using it on their own people (because of the protests?) and blaming it on another country. Trying to get their people to support the war? Could have started being spread in vaccines or a new type of shot... or even the drinking water if it's that type of virus. If this is the future it could be spread in many different ways, right?
Hmmm... the government could have intended to use the virus on a waring country but it slipped out somehow... Someone being tested on made a run for it, a doctor didn't follow procedure, or a cadaver not being properly disposed?
Thoughts?

I must go get ready for school. I'll be back in about ten - twelve hours. Maybe sooner.