Is there any risk of illness from eating lettuce?

Yes. Both packaged, pre-cut lettuce and unpackaged lettuce may be contaminated with the pathogens E. coli or salmonella, depending on how the lettuce was farmed, harvested or packaged. Pre-cut lettuce has also been associated with outbreaks of Listeria monocytogenes, another food-borne pathogen. Since any of these pathogens can cause serious illness, careful storage, handling, and preparation of lettuce is a must.
 
Lettuce also is prone to spoilage. It is not advisable to eat lettuce showing visible signs of spoilage, including wilting and excessive browning. Spoiled lettuce also may assume a slimy appearance or texture.
 
Lettuce can also turn brown as a result of oxidation. In packaged (pre-cut) lettuce, oxidation causes moisture to collect at the bottom of the bag. The water spawns mold that can make you sick. The same process can --and does--occur with heads of lettuce that are wrapped for purposes of storage. However, unwrapped lettuce dries out more quickly. It also may pick up refrigerator odors.  The solution is to wrap a head of lettuce loosely.
 
Source(s):
Medicine.net "Tips For Keeping Your Produce Safe"

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