I believe the "level editor" for Z-Virus was a paint program; If you look at the LevelData directory, you'll see nothing but bmp "relief maps". The game (presumably) loads in the image for the correct map and creates a mesh for it, the brightness of each pixel denoting the altitude of the vertex. Certain colours (various shades of red) denote objects to be placed on the map, presumably at the same altitude as a neighbouring pixel.