Describe why there is a difference between the number of reported cases of foodborne illness and the number of estimated cases.

Irrespective of the surveillance system, many cases of foodborne illness are not reported because:

the ill person does not seek medical care,
the health-care provider does not obtain a specimen for diagnosis,
the laboratory does not perform the necessary diagnostic test, or
the illness or laboratory findings are not communicated to public health officials.

Also, infections caused by some microorganisms are not reportable and so may not be included in summary accounts of foodborne illness.

Therefore, to calculate the total number of illnesses caused by each pathogen, the authors of the CDC-sponsored report "Food-related illness and death in the U.S." deemed it necessary to account for underreporting, i.e., the difference between the number of reported cases and the number of cases that actually occur in the community. For Salmonella, a pathogen that typically causes nonbloody diarrhea, the degree of underreporting has been estimated at approximately 38 fold. For E. coli O157:H7, a pathogen that typically causes bloody diarrhea, the degree of underreporting has been estimated at approximately 20 fold. Because similar information is not available for most other pathogens, CDC used a factor of 38 for pathogens that cause primarily nonbloody diarrhea (e.g., Salmonella, Campylobacter) and 20 for pathogens that cause bloody diarrhea (e.g., E. coli O157:H7, Shigella). For pathogens that typically cause severe illness (i.e., Clostridium botulinum, Listeria monocytogenes), CDC arbitrarily used a far lower multiplier of 2, on the assumption that most cases come to medical attention. More detailed information describing the methods of accounting for underreporting are included in the below on-line document.

Source:
Mead, P.S., L. Slutsker, V. Dietz, L.F. McCaig, J.S. Bresee, C. Shapiro, P.M. Griffin, and R.V. Tauxe. 1999. Food-related illness and death in the U.S. Emerging Infectious Diseases 5:607-625. Complete publication available on-line at http://www.cdc.gov/ncidod/eid/vol5no5/mead.htm.