Any cutting board can become dangerous to your health if you let it collect bacteria.
Rule number one is to use a different cutting board for food that will not be cooked than the one you use for raw meat and fish. If raw vegetables are contaminated, there is no heat process to kill bacteria after you use the cutting board.
All cutting boards should be cleaned regularly with either salt or a bleach solution.
Now the debate. It would seem that plastic cutting boards would be easier to clean, and most research shows them to be the safer of the two. But recent research has shown that the knife scratches harbor bacteria that is not easily cleaned out of the grooves. Hence wooden boards may in fact be safer because the grooves are not as deep.
I have found that a good sharp paint scraper can successfully scrape all the knife marks out of both plastic and wooden cutting boards.
So my conclusion is to clean cutting boards after each use by rubbing in salt (it doesn't taste or smell as bad as bleach), scrape them down to a fresh surface regularly, and keep wooden boards oiled with mineral oil (from the drug store) -- it is food grade but will not go rancid. If you do all this, you can use either plastic or wood. Remember that cracks in the wooden boards definitely harbor bacteria. They should be re-glued or replaced.