Barstow explains:

“So the laws requiring that these diseases be reported make it possible to collect information about disease frequency. [See the Law Library]. But that really isn’t enough. Obviously, you have to do something with the data in order to identify outbreaks. In the case of hepatitis A we generally see 12-15 cases of hepatitis each year in Sparta scattered throughout the year. Whenever a case occurs we investigate to try to ascertain the source and to prevent any additional spread. If it involves a diapered child, or a day-care attendee, or a food handler we want to prevent further transmission. These sporadic cases tend to be the result of travel outside the US, or from household or sexual contact with an infected person; about half the time we never figure out what the source was."

"In this case, we were first alerted when the emergency room physician reported the occurrence of hepatitis A in the 31 year-old woman. Over the next several days three other local physicians called the board of health and reported four more cases, so we began to think that this was more that the usual sporadic cases. As a matter of routine, we keep an ongoing bar chart for all reportable diseases, and we update this at least once a month, or more frequently if there is suspicion of an outbreak."