bad Oyster makes ill

I order 6 oysters when they were served on the surface they appeared ok. I continued to consume one oyster just to realise it had gone rancid. I had a better look and upon inspection the other 5 were sticky, mushy , milky consistency. The oyster then a few minutes later started to make me ill which had me running to the bathroom. How does an established serve rancid oysters, where is the quality control? I travelled all the way from Bellville which was a near impossible journey home as I was running to the bathroom constantly