Several years ago, epidemiologists noted that the number of cases of salmonellosis increased dramatically in the summer and was lower the rest of the year. In contrast, the number of cases of typhoid...

Several years ago, epidemiologists noted that the number of cases of salmonellosis increased dramatically in the summer and was lower the rest of the year. In contrast, the number of cases of typhoid fever, caused by the same bacterium, was constant throughout the year. Explain these observations.