red tide is a toxic algae that blooms under certain water conditions - the algae is actually a living body that can make humans very sick if not kill them - mollusks - such as oysters - clams - mussels etc - are more susceptible to carrying the toxins due to their feeding methods - they filter the water for the nutrients they feed upon. those toxins can stay in the shellfish for varying lengths of time - so to assure safety many harvest areas become closed to prevent the mollusks from reaching market.
as for only eating oysters in months with an "R" - it's a wives tale - those months are considered safe because the months w/o an "R", May, June, July & August, are high "bloom" months when red tide is more prevelent.
THE OTHER RED TIDE!!! &
MORE!!!