Misfires On left side of engine

Hi, I have a 98 Caravan with the 3.0 in it. It run a little rough while idling and I can feel it missing at about 40-50 mph. The check engine light comes on. I checked the codes and the codes are PO300, 301,303, and 305. Those are mis-fire codes for cylinders 1, 3, and 5. Those are the cylinders that are on the left side of the engine. Does anyone know what could cause just the cylinders on one side of the engine to mis-fire? Thanks.