Is there any risk of illness from eating a sandwich made with deli (sliced luncheon) meat?

A pathogen called Listeria monocytogenes can contaminate deli meats and cause illness.  This pathogen is killed by pasteurization and cooking. However, contamination can sneak in after the product is heated but before it’s packaged. Unpackaged meats can also be contaminated in supermarkets or delis as a result of  unsanitary  handling during slicing.
 

Listeria is most likely to be a problem in cured meat products such as hot dogs and bologna for these reasons: 1) it can grow slowly in refrigeration; 2) unlike most bacteria, it tolerates the sodium nitrite used to "cure" the meat; and 3) it likes only a very small amount of oxygen in its environment, so it thrives in the vacuum-packing of many cured meat products.

Groups most at risk of contracting listeriosis include people over 60, people with weakened immune systems, unborn babies and infants, and pregnant women. People at greater risk should reheat deli meats to a temperature of 165°F immediately before consuming them.  Healthy adults and children only occasionally get infected with Listeria, and, if they do, they rarely become seriously ill.
 
The bacteria Stapphylococcus aureus--better known as staph--is also associated with deli meats, usually as a result of poor handling in the home.  Thorough handwashing before handling these products will minimize the possibility of contaminating them. 
 
As a result of oxidation, deli meats are also subject to bacterial spoilage. These bacteria generally aren't harmful. They simply result in a product that looks and tastes inedible.
 
Source(s):
Maple Leaf Foods "Listeria and Deli Meat"
Safefood.net "Staphylococcus aureus"
Foodliabilitylaw.com "2009 Priorities for USDA in Food Safety"
CDC: Division of Foodborne, Bacterial and Mycotic Diseases Listeriosis