dishwasher problem - water coming?

We are not using the dishwasher a lot. After a month or more, I opened the dishwasher and found that water was filled up in the bottom of the dishwasher. Actually it overflowed to the floor. We moved in four months ago. We have used it three or four times and every time we found water in the botom but was not much. What would be the problem and how can I fix it? Thanks.

Inside the dishwasher at the bottom is a food screen that can plug check this first.

Thanks, CASS. When it finish the operation, I don't see that much water. But after a while not using it, like today- actually first time- I saw a lot of water filled that overflows to floor even I did not run. And there is quite odd smell. At the summer I thought it is just because of the hot weather. But I still smell it. What do you think the reason of those?

question

do you have an air gap [plastic vent, on counter ,of faucet base] or is the drain hose looped up to the bottom of counter ALL THE WAY?
do you have a garbage disposal? some times the plastic plug is not knocked out at the hose disposal end by the installer! water can't drain