In this passage, we learn that immigrants were held at Ellis Island while they were physically examined. If the doctors found a person had a communicable disease, they were hospitalized in the island. This means that the immediate cause of hospitalization was being diagnosed with a disease. This was most likely a measure that fulfilled two purposes. On the one hand, it provided treatment to sick people. On the other hand, it tried to prevent the spread of contagious diseases to the population at large.
