Microbial pathogens in food cause between 6.5 million and 33 million cases of human illness and up to 9, 000 deaths each year in the United States. Over 40 different foodborne pathogens are believed to cause human illness. The annual cost of human illness caused by seven foodborne pathogens for which we have estimates ranges between $5.6 billion and $9.4 billion. Meat and poultry are the primary sources. Microorganisms are commonly found in soil, water, plants, and animals. Most do not cause human illness. In fact, we rely on some microorganisms in the making of bread, alcohol, cheese, vitamins, and antibiotics. Some, however, do cause human illness. Pathogens-microorganisms that cause disease-include viruses, bacteria, parasites, and fungi. The bacterium Staphylococcus aureus lives harmlessly on human skin and in nasal cavities of less than half the people in the United States, but in food it can produce toxins that cause human illness. Another bacterium Escherichia coli O157:H7 usually lives harmlessly in the intestinal tracts of some cattle, but in people it can cause serious illness, including bloody diarrhea and kidney failure, as well as premature death. People can acquire the bacteria by eating mishandled or insufficiently cooked meat from infected animals. Half of all foodborne illnesses have no identified cause. Yet those foodborne illnesses that are confirmed and reported to the Centers for Disease Control and Prevention (CDC), over 90 percent are attributed to bacteria. Increasing Government scrutiny over food safety is improving efforts to restrict microbial pathogens in the food supply-as well as to improve data on the numbers of cases and costs associated with these pathogens. This article is the first in a new Economic Research Service series that will track the estimated costs and incidence of seven foodborne diseases over time (see box on key foodborne pathogens). These diseases were chosen for this series because they are commonly found in meat and poultry. Public-health officials can compare the cost-of-illness (COI) estimates to identify the most expensive foodborne pathogens and illnesses COI estimates can also be compared with the costs of pathogen-control programs to determine what level and direction of intervention may be needed.