Why do some bacteria make people sick?

By Susan Brewer, Ph.D., University of Illinois,
Department of Food Science and Human Nutrition
 

Bacteria don’t set out to cause illness. They are simply taking advantage of conditions in which they can grow, or they are producing substances which prevent other organisms from taking over their environment (food, space, etc.).  Bacteria that cause infections can set up housekeeping inside our bodies because the climate is just what they like, and the nutrients are there.  Our bodies react to them as “foreign invaders” (because they are)  and send out the immune system cells to get rid of them. The results are fever, body aches, and fatigue.  Bacteria that cause poisoning (intoxication) produce substances to discourage other organisms from competing with them.  When consumed, these toxins are powerful irritants to various parts of the body, mostly the gastrointestinal tract (so they cause vomiting and/or diarrhea) or the nervous system (so they cause paralysis, loss of feeling, and compromised breathing).
 
Source(s):
 
Kendall, P. 2008.  “Bacterial food-borne illness.”  Cooperative Extension Service. Colorado State University, Fort Collins, CO.  Fact sheet No. 9.300.
U.S. D.A. Food Safety and Inspection Service. 2006. “Basics for Handling Food Safely.”