species of the genus Salmonella are found in the intestinal tract of birds (e.g., chickens - hence its rare presence in eggs), pigs, and cows - among others. it may or may not cause any symptoms in those animals. in addition some humans may be carriers and exhibit no symptoms (e.g., "Typhoid" Mary) and end up infecting others. so, lack of cleanliness and/or poor hygiene contribute to acquiring Salmonellosis food intoxication.
organic fertilizers (e.g. manure), wild animals, asymptomatic humans, contaminated processing machinery can all contribute to the contamination of the produce.
common sense: thoroughly wash all fruits and vegetables whether home grown or store bought. cook all eggs and foods w/eggs.