Multinucleated sphere unknown (MSX) is a disease of eastern oysters (Crassostrea virginica) caused by the spore-forming parasitic protozoan Haplosporidium nelsoni. MSX does not affect humans however it can cause high mortality in oysters. H. nelsoni’s ability to infect oysters and the rate it causes mortality is heavily influenced by salinity. H. nelsoni needs a salinity of 15 ppt or greater to infect oysters. At a salinity of 10 ppt or lower though, oysters are able to expel H. nelsoni from their tissues. Using this information, salinity can thus be used as an indicator of whether MSX prevalences will increase or decrease in areas where salinity fluctuates around the 15 ppt threshold. The amount of mortality that results from MSX however is difficult to predict as there are many factors at play that influence this. To better understand the relationship between MSX and oyster mortality as well as the details of salinity’s impact on MSX, oysters were sampled from 3 sites in the Maryland portion of the Chesapeake Bay. The sites were the Choptank River (Lighthouse bar), Choptank River (Royston bar), and Tangier Sound (Piney Island bar). They were sampled for MSX (n=30 per site) in Fall 2023, Spring 2024, Summer 2024, and Fall 2024 so the seasonal cycle of MSX could be captured, and effects of low and high salinity could be seen. These sites were also tested for Dermo disease so this could be taken into account as a possible source of mortality if present. Dermo is also harmless to humans though it can be lethal to oysters if infections are heavy. Dermo is usually less of a concern for oyster farmers than MSX though since oysters are usually harvested before dermo infections have time to become lethal. The data obtained in this study along with salinity data records will be helpful in predicting when a high MSX prevalence season for the Maryland portion of the Chesapeake Bay is likely ahead.