In this case, the likely reservoir for the Salmonella enterica serotype Typhimurium infection is the intestinal tract of infected animals, particularly poultry such as chickens. Salmonella can also be found in the environment, such as in contaminated water or soil, and can survive on surfaces for extended periods of time.
In this specific case, it is possible that the eggs used to make the cookie dough were contaminated with Salmonella bacteria from the chicken that laid the egg. Raw or undercooked eggs have been known to be a source of Salmonella infections. It is important to note that while the risk of Salmonella infection from eggs is relatively low, it is still present, and the CDC recommends cooking eggs to an internal temperature of 160°F (71°C) or higher to reduce the risk of infection.