Normal sudoku rules apply. All cells are either a zombie or a human. No 2x2 area of cells are just zombies or just humans. If a cell is orthogonally connected to a zombie cell with a higher digit it becomes infected and becomes a zombie. If a cell with a 9 is orthogonally connected to a zombie cell with a 1 it becomes infected and becomes a zombie. The cell with the circle is “patient zero”. It is a zombie. The infections of all the other zombies in the grid can be traced back to patient zero.