Non-Boss infected are this game's versions of the Special Infected from Left 4 Dead (the creators of which also made this). I would state which ones to watch out for but honestly any enemy does does not look like a traditional zombie and has no health bar counts towards this achievement. Glowing red spots are also a very visible weakness. You should get this just by going through the first few levels.
There are specific enemies that can show up in certain maps that do not count for this achievement, although it will have popped long before you encounter them.