 It has been known for more than a century that consumption of raw or insufficiently cooked shellfish can be associated with infectious diseases . These are caused either by bacteria naturally present in the sea such as Vibrio spp. or by viruses and bacteria from sewage effluent that sometimes contaminates coastal waters . As filter feeders , bivalves pass large quantities of water through their gills , filtering out the organic particles , including the microbial pathogens . These are retained in the animals ' tissues and become concentrated in their liver @-@ like digestive glands . Another possible source of contamination occurs when bivalves contain marine biotoxins as a result of ingesting numerous dinoflagellates . These microalgae are not associated with sewage but occur unpredictably as algal blooms . Large areas of a sea or lake may change colour as a result of the proliferation of millions of single @-@ cell algae , and this condition is known as a red tide . 
