Salmonella bacteria live in the intestines of birds, animals, and humans. Most human infections are caused by eating food or drinking water that has been contaminated by feces.

Foods most commonly infected are:
Uncooked meat, seafood and poultry: Contamination most commonly occurs during the slaughtering process. Harvesting seafood in contaminated waters is a common cause.

Uncooked eggs: Eggs from an infected chicken may contain bacteria. The U.S. Food and Drug Administration (FDA) estimate that, every year, there are 79,000 cases of foodborne illness in the U.S., due to eating eggs containing salmonella. Raw eggs are found in some types of mayonnaise and homemade sauces.

Fruits and vegetables: These may be contaminated if they have been watered or washed in contaminated water. Fruits and vegetables can become contaminated if a person handles raw meat and then touches the fruit without washing their hands.

Other causes include:
Lack of hygiene: Common causes of contamination and infection include kitchen surfaces that are not kept clean, and not washing hands during food preparation, after using the bathroom, or after changing a baby's diapers. A person with contaminated hands can pass the infection on to other people by touching them, or by touching surfaces that others then touch.

Keeping pet reptiles or amphibians: Most reptiles and amphibians carry Salmonella in their gut without becoming ill. They shed the bacteria in their droppings. These can quickly spread onto their skin and then anything they come into contact with, including cages, toys, clothes, furniture, and household surfaces.

Risk factors
At the highest risk are children, older people, and and those with a weakened immune system, due, for example, to HIV or AIDs, cancer or cancer treatment, and some other conditions.

Young children should not be allowed to handle reptiles or chicks and young birds. Breastfeeding is the safest type of nutrition for young infants.

During pregnancy, complications include dehydration and bacteremia, or bacteria in the blood. This can lead to meningitis. Salmonella can also pass to the fetus. The infant may have diarrhea and fever after birth and a risk of developing meningitis.