outbreak [out-breyk]

The occurrence of illness or disease in a specified location and time period that is greater than the expected rate.

During the 2009 flu pandemic, I spent a day observing a team of medical investigators track how the new bug sped through an elementary school in Berks County. They marked a whiteboard with telltale boxes and arrows showing 16 sick kids in one fourth-grade classroom but only five in another. Had they sneezed on each other? Shared lunch?

In an instant, I was hooked on epidemiology - the study of the patterns and causes of diseases. It is epidemiologists, embedded in universities and health departments around the world, who unravel and stop oubreaks of salmonella, fungal meningitis, and hundreds of other infectious diseases, saving the lives of millions.

Contagion offered a Hollywood version of what they do. Now you can try it yourself using the nifty new iPad app Solve the Outbreak.

Its three fictional outbreaks were created by the Centers for Disease Control and Prevention, based on real-life events. I randomly chose one called Midterm Revenge (76 sick, three hospitalized).

Clue No. 1: A couple of university students in Texas, feeling miserable after a night of partying and pizza, call the health center seeking midterm exam extensions. Dozens more soon show up with vomiting and diarrhea. What to do?

I said keep the sick kids together until their symptoms are gone. This turned out to be the wrong answer - and the only time that the app, as far as I could tell, had not given me the necessary information to get it right.

Over the course of the investigation, it told me how many had which symptoms; that the health center suspected a stomach bug; and that onset of symptoms spanned a four-day period (used to create a helpful bar graph known as an epidemic curve). Case descriptions, data, and tips are an added click with each clue.

Next task (correctly chosen): Interviews of both sick and well students to find out what they had in common. Data plotted on pie charts were striking: Sick students were males and females, all class levels, four dorms. But 100 percent ate at just one of the three cafeterias on campus. And most had sandwiches.

"Hint: To identify the likely source of the outbreak, look for a high attack rate" - the number of people who ate a food item and are sick divided by the total number of people who ate a food item - "among those who ate a specific food and a low attack rate among those who did not eat this food."

Attack rates shown on a data page confirm the sandwich area as the source. But the rates for specific items - mayonnaise, ham, cheese, turkey, mustard - are all over the place.

A tip is really helpful. In an interview, the woman in charge of making sandwiches recalls having diarrhea and stomach cramps a few days before the outbreak started. Her hands are constantly touching things. There are no employee-hand-washing signs posted.

Bingo! Lab tests back from the local health department's virology lab confirm norovirus, a highly contagious gastrointestinal infection that lasts one to three days.

It is a perfectly plausible scenario. Philadelphia has had 11 institutional outbreaks of norovirus this season, according to the health department, and ill food handlers are a common source; kitchens can be cited for lack of hand-washing signs.

The app offers a bunch more information, including "Disease Detective Camps" for middle and high schoolers. Appropriate photos (a college student slumped next to a toilet bowl) enliven the exercise. CDC says more fictional outbreaks are in development.

Meanwhile, having earned my "novice" chops on the miserable but limited Midterm Revenge at a university in Texas, I'm ready to go abroad: Up Sick Creek, an epidemic in Kenya, sickened 404 people, hospitalized 115, and killed 118. That's a serious outbreak to solve.

Contact Don Sapatkin at 215-854-2617 or dsapatkin@phillynews.com.