Wood has anti-microbial properties, which plastic does not. The other commonly raised concern is contaminants getting stuck in the grain or cut grooves of wood cutting boards. The problem is any plastic cutting board will also have grooves after it has been used for a while. Ultimately, a properly cleaned wooden cutting board is probably safer than a properly cleaned plastic one.

On the topic of anti-microbial materials, silver, copper, and copper alloys including brass are anti-microbial, so the best material for doorknobs and other high-contact surfaces from a sanitary standpoint is uncoated silver or brass.