Causes and Risk Factors of Salmonella

A salmonella infection (salmonellosis) can be contracted from food, pets, or exposure to human or animal feces. Children, the elderly, and people with a weakened immune system are most at risk. Learn about the common causes and risk factors so you can prevent this source of food poisoning and diarrhea.

Salmonella risk factors
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Common Causes

Salmonella infection is caused by bacteria in the genus Salmonella, which live in the intestinal tract of humans and animals and is spread through the feces. While this bacteria may not make an animal sick, it can sicken humans. A person infected with salmonella can spread it to other people through the feces.

Nontyphoidal salmonella leads to the typical salmonella gastroenteritis infections. Typhoid types produce typhoid fever, which is uncommon in the U.S. but can be seen in developing countries. Several different serotypes (or distinct variations) of this bacteria are commonly isolated in outbreaks and infections.

There are two main ways salmonella is spread: through contaminated food and water and contact with animals who carry the bacteria.

Foodborne Salmonella Infection

Salmonella bacteria are present in the feces of many animals, including beef, poultry, and fish, and often contaminates their meat, milk, or eggs. In the case of eggs, bacteria can be present inside the shell as well as outside.

Fecal contamination of water or cross-contamination during processing or food preparation can lead to spreading the bacteria in vegetables, fruit, seafood, spices, and processed foods. Cooking will kill the bacteria, which is why meat thermometers are used when cooking poultry. Pasteurizing milk and boiling water also kills the bacteria.

Contact With Animals

You can be exposed to salmonella by farm animals and pets. If you work on or visit a farm or keep barnyard farm animals, you can contact the bacteria as it contaminates their enclosures, fur, feathers, and the groundwater. These animals can appear clean and healthy and still transmit the bacteria.

The animals that have been known to spread salmonella include poultry, goats, cattle, sheep, and pigs. While you may not think your backyard chickens could be a source of this bacteria, ​the Center for Disease Control (CDC) reported over 790 cases of salmonella were spread that way in the first half of 2017. By comparison, only 53 outbreaks were confirmed from 1990 to 2014. Farming practices associated with salmonellosis risk include:

  • Failing to collect eggs at least daily, especially in warm weather
  • Not refrigerating the eggs right after collection
  • Not washing hands after touching or cleaning a chicken 
  • Kissing or petting the chicken 
  • Failing to quarantine visibly ill chickens
  • Fertilizing gardens with fresh, uncured manure which can contaminate crops

Pets are also a source of salmonella. Reptiles such as iguanas, lizards, and turtles often have the salmonella bacteria on their outer skin or shell. Turtles and other reptiles with salmonella aren't themselves sick and don't have any symptoms. Pet birds such as parakeets and parrots, rodents such as hamsters and guinea pig, amphibians such as frogs and toads, hedgehogs, dogs, cats, and horses can be sources. Besides touching the animal, you can pick up the bacteria from their cage, tank water, bedding, food, or toys.

Handling wild animals can also transmit the bacteria. Free-living turtles were once thought not to be as great of a risk, but it's now known that wild turtles may carry salmonella, or they can acquire it if you make them a pet. Other animals you might handle include wild frogs, toads, mice, rats, and birds.

Contact With Humans

People who have a salmonella infection will shed the bacteria in their feces. Those who have had diarrhea should not return to child care, school, or work until 24 hours have passed. If they handle food as part of their job, they should not return to work until 48 hours have passed without symptoms.

In some locations, food handlers can't return to work until testing shows they are free of the bacteria. Even after they feel well again, some people continue to carry the bacteria and shed it. They can contaminate surfaces and spread the germs by hand if they do not wash well after using the bathroom.

Risk Groups

Small amounts of bacteria may not produce a salmonella infection. However, infants, children under age 5, people over age 65, and those with compromised immune systems are more likely to get a salmonella infection after exposure. Specific conditions and medications that weaken immunity to salmonella infection include AIDS, sickle cell disease, malaria, corticosteroids, and anti-rejection medications.

People who take antacids are at greater risk as more ingested bacteria survive to reach the gut. Those with inflammatory bowel disease are at risk due to the damage to the intestinal lining. You may also be at higher risk after taking antibiotics as the friendly gut bacteria have been killed, leaving that habitat open for salmonella.

Lifestyle Risk Factors

There are many things that increase your risk of contracting or spreading salmonella. Here are habits and practices to avoid:

  • Failure to wash your hands after using the restroom, diapering, or caring for a sick person
  • Failure to wash your hands before and after preparing food and after handling raw meat or eggs
  • Handling or cutting raw meat or eggs and then using the knife, cutting board, bowl, or other utensils for other food items such as vegetables or fruit
  • Consuming undercooked or raw meat, eggs, or unpasteurized milk products
  • Not washing fresh fruit and vegetables before eating them

Pet Amphibians, Reptiles, and Live Poultry

Amphibians, reptiles, and live poultry carry the biggest risks as pets. These include turtles, lizards, frogs, and chickens.

The animals listed above should not be kept in a home with:

  • Children under age 5
  • People over age 65
  • People who have immune system problems

These pets should also not be kept in facilities that serve these age groups, such as:

  • Daycare
  • Hospitals
  • Senior centers
  • Skilled nursing facilities

People in these risk groups should not touch these animals. They should avoid water that has been touched by these animals, such as tank water from their enclosures or water used to wash the pet.

All children and adults should avoid eating or drinking around pets in this group. You also shouldn't eat or drink in the room where the pet's cage or aquarium is located or where the pet has been allowed to roam.

All Pets

These behaviors increase your risk of getting salmonella from a pet:

  • Allowing animals in areas where food and drinks are prepared, eaten, or stored
  • Failure to wash your hands after handling or touching an animal, especially before preparing or eating food
  • Discarding cleaning water from the pet's habitat in a sink that is used for food preparation
  • Allowing people who are at increased risk for salmonella to clean a pet's habitat, especially without wearing disposable gloves
  • Using uncured pet manure to fertilize gardens or flower beds

Frequently Asked Questions

  • What causes salmonella in eggs?

    Live poultry can carry salmonella bacteria and contaminate the inside of the egg before the shells form. The outside of the shells may also have salmonella from poultry droppings or the environment where the eggs are laid.

  • What diseases does salmonella cause?

    Most types of salmonella bacteria cause salmonellosis, an infection that causes gastroenteritis. Another type, Salmonella Typhi, causes typhoid fever, while Salmonella Paratyphi causes paratyphoid fever, a similar illness. Typhoid fever and paratyphoid fever are more common in areas of South Asia, especially Pakistan, India, and Bangladesh.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Salmonella.

  2. Giannella RA. Salmonella. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston, TX: University of Texas Medical Branch at Galveston.

  3. Crum-Cianflone NF. Salmonellosis and the gastrointestinal tract: more than just peanut butterCurr Gastroenterol Rep. 2008;10(4):424-431. doi:10.1007/s11894-008-0079-7

  4. Centers for Disease Control and Prevention. Multistate Outbreaks of Human Salmonella Infections Linked to Live Poultry in Backyard Flocks.

  5. Basler C, Nguyen T, Anderson TC, et al. Outbreaks of Human Salmonella Infections Associated with Live Poultry, United States, 1990–2014Emerging Infectious Diseases. 2016;22(10):1705-1711. doi:10.3201/eid2210.150765

  6. Weir E, Doré K, Currie A. Enhanced surveillance for Salmonella NewportCMAJ. 2004;171(2):127-128. doi:10.1503/cmaj.1041038

  7. Centers for Disease Control and Prevention. Salmonella and eggs.

  8. Centers for Disease Control and Prevention. Typhoid fever and paratyphoid fever: Questions and answers.

Additional Reading
  • Salmonella Infection. CDC.

  • Salmonella Questions and Answers. USDA Food Safety and Inspection Service.

  • Salmonella Infection. Mayo Clinic.

By Vincent Iannelli, MD
 Vincent Iannelli, MD, is a board-certified pediatrician and fellow of the American Academy of Pediatrics. Dr. Iannelli has cared for children for more than 20 years.