What are the most common ways produce becomes contaminated?

Produce such as spinach, lettuce, tomatoes, sprouts and melons can become contaminated with Salmonella, Shigella, or  E. coli O157:H7. Contamination can occur during growing, harvesting, processing, storing, shipping or final preparation. Sources of produce contamination are varied as these foods are grown in soil and can become contaminated during growth or through processing and distribution.

http://www.niddk.nih.gov/health-information/health-topics/digestive-diseases/foodborne-illnesses/Pages/facts.aspx#2

Farmers work diligently to apply organic matter (manure used for fertilizer) during early stages of growth to reduce the potential for contamination.

http://digestive.niddk.nih.gov/ddiseases/pubs/bacteria/#9