A risk assessment is a process of determining the probability of occurrence of adverse health effects resulting from exposure to a hazard. There are four steps: 1) hazard identification; 2) hazard characterization; 3) exposure assessment, including a dose-response assessment; and 4) risk characterization. Ideally, for harmful algal blooms, exposure assessments should begin with toxigenic plankton in the sea and end with a probability of illness after ingestion of a seafood containing toxin. Microbiological risk assessments have been developed with models from an animal host through to the consumer, where there is limited information to connect the reservoir with the hazard in the food. One of these is suggested as a model to build upon for algae. The hazard is Vibrio vulnificus, a bacterium that grows well in warm seawater. Oysters in the Gulf of Mexico which concentrate this organism through filter feeding have been implicated in illnesses. The assessment considers information from the Vibrio in the sea to the ingestion of an oyster meal. A model was developed to consider the prevalence, numbers and seasonality of V. vulnificus in oysters, and the influence of meal sizes. From these data, a simulation model, using 10,000 iterations, computed an average probability of illness in a healthy individual of 6.6x10^-6 from eating a single raw oyster harvested during the warm summer months, and 6.7x10^-10 in the cooler winter and spring. Assumptions have been made based on existing knowledge and an allowance has been made for uncertainty. The bacterial model would have to be adapted for specific algal situations.