This was pretty buggy when I got it. Infected soldiers are, you guessed it, zombie soldiers in uniform. They can be found in and around military bases/camps. It took around 7 or 8 kills to get this to trigger for me.
Theres an app on mobile devices called "iZurvive - Map for DayZ" which you can use to find these camps/bases. Be careful though, as other players frequently farm these spots for assault rifles and other gear.