Plastic cutting boards have a nonporous surface that provides no place for bacteria to dwell. However, bacteria can just as easily live on the surface and after using the board for a while, your knife will probably chew up the fine surface of the board providing plenty of hiding spaces for bacteria to survive even through vigorous washing. This is troublesome to deal with and it is wise to scrub the cutting board down immediately after using.  .... Wood cutting boards deal with bacteria in the opposite way that plastic boards do.  Wood boards actually absorb the bacteria into the wood. After the surface of the wood has been cleaned and dried, the bacteria near the surface dies. It turns out the wood near the surface forms a hostile environment for bacteria to live in. There are lots of bacteria living in the cutting board, but about 1/8 in. below the surface. This is deep enough that a heavy handed chop into the wood is unlikely to release bacteria (unless the wood splits). If your cutting board fits in your microwave oven, heating up the board in the microwave for 30 sec. or so will completely cleanse the board of bacteria, inside and out. As with plastic boards, prop a corner up to keep moisture from collecting.