Prototype

Unfortunately, you will have to do some serious grinding to unlock this trophy. After two run throughs (second using New Game + of course), completing all of the kill and war events, and going for other trophies where killing infected is involved, I only had around 18,500 infected kills. So try to save this one for last so that you already have some of the kills under your belt. When it comes down to this one, the general best strategy is to pick a kill event where you can rack up many infected kills quickly and keep playing it over and over. The most obvious would be the Rolling Thunder event, where you get two minutes in the Thermobaric Tank to kill as many infected as possible. This event has the highest number of points required for the Platinum medal (750) of any kill event making this the optimal event to grind for kills.

You can track your progress under the Statistics -> Infected page on the pause menu, but there is no total "Infected Killed" statistic, so you will have to put on your mathematician's hat and do a bit of addition.