Two reports recently issued by the CDC (Centers for Disease Control) deal with food and how it can make people sick. Perhaps the most striking finding is that nearly 1 in 6 Americans get sick every year from the food they eat. Roughly 80% of these illnesses are likely caused by unknown agents — microorganisms or chemicals.

The reports attribute 3,000 deaths and 128,000 hospitalizations a year to foodborne illness.

Much of protecting against foodborne illness is common sense: cleanliness, proper refrigeration, cooking food thoroughly, making sure that utensils that touch raw food do not touch cooked food.

Estimating foodborne illness is tricky. The chief sign of foodborne illness is stomach or intestinal distress: gastroenteritis. But not all cases of gastroenteritis come from eating tainted food. And not all cases get reported, so there is quite a bit of uncertainty in these figures. This creates difficulties both in determining how many cases of gastroenteritis occur yearly and how many are caused by eating tainted food.

To estimate the number of cases of an infrequently reported disease that is caused by unknown agents, researchers took the percentage of all cases of gastroenteritis with known foodborne causes and assumed that the same percentage of gastroenteritis cases of unknown cause were also due to tainted food. It's purely a guesstimate, but it's currently the best available. The final estimates are 9.4 million cases of food illness annually from known pathogens and 38.4 million suspected cases from unknown pathogens or chemicals.

There are two major causes of foodborne illness: toxins produced by microorganisms and infections. Collectively, they're often called food poisoning. Some microorganisms (usually bacteria) produce toxins, harmful chemicals that contaminate food, and eating this food can make people sick even when the food contains no living bacteria. Botulism is an extreme example of this. Infections are caused when live pathogens in food are ingested and grow inside of the body, as happens with Salmonella. This distinction is unlikely to matter much to someone experiencing stomach cramps.

While there have been a few high-profile Salmonella cases annually in recent years, most food poisoning occurs under the radar. The vast majority of cases do not require hospitalizations, but all are unpleasant. People sometimes have a case of food poisoning but attribute it to upset stomach or stomach flu.

There are many sites at which food can become contaminated: on the farm, during transportation, in warehouses, supermarkets, restaurants or in the home. Because many people do not grow or even prepare their own food any more, there's not that much they can do to protect themselves from food that's tainted before it enters their house. But there is a lot they can do once it enters the house.

Much of protecting against foodborne illness is common sense: cleanliness, proper refrigeration, cooking food thoroughly, making sure that utensils that touch raw food do not touch cooked food and keeping raw foods separated from cooked foods and hot foods separated from cold foods.

The estimate of 48 million yearly cases cannot be compared to the CDC's last estimates from 1999 to tell if foodborne illness is rising or declining. The 1999 figure of 72 million annual cases is also an estimate and was made using a totally different set of mathematical assumptions. Comparisons would be more meaningless than comparing apples to oranges. A different type of data, trend data, would be required to make a meaningful estimate of the rise or fall of foodborne illness over the last decade.

But not all the information in the reports is of the head scratching variety.

Salmonella was the leading cause of hospitalization (35%) and death (28%) due to foodborne illness.

Nearly 60% of all known foodborne illness was caused by norovirus, though it caused fewer hospitalizations (26%) and deaths (11%) than Salmonella.

About 90% of all known foodborne hospitalizations and deaths were caused by seven organisms: Salmonella, norovirus, Campylobacter, Toxoplasma, E. coli O157, Listeria and Clostridium perfringens.

Specific information on how to avoid foodborne illness is available at

The CDC reports appear in the December 2010 issue of Emerging Infectious Diseases, the CDC's own peer-reviewed journal and will also appear in the January 2011 issue. All articles in the journal are freely available.