Foodborne bacteria and parasites, such as tapeworms, giardia, and ascaris may cause serious health implications. They are hard to see. But, key preventative measures can help keep you safe.

Left unchecked, foodborne parasites and bacteria can be deadly, and controlling them is a matter of serious concern for governments and food producers alike.

You typically can’t see or taste foodborne parasites or bacteria, which can be a little unsettling. Here are eight that could be lurking in a meal, along with some straightforward methods for preventing illness and infection.

When we talk about the E. coli that makes humans sick, we’re usually talking about Shiga toxin-producing E. coli, or STEC. Strains of STEC, most commonly 0157, create a toxin called Shiga that’s dangerous to people.

E. coli is generally found in undercooked beef.

You can’t see it, taste it, or smell it. If you ingest it, you could experience stomach cramps, vomiting, fever, and diarrhea — often bloody.

How to prevent it

There are no medications available to help treat an E. coli infection and no vaccines that can prevent it. You can lower your risk by cooking all of your meat thoroughly until it reaches an internal temperature of 160°F (71.1°C).

When preparing beef, keep your work surface clean, wash your hands frequently, and don’t cross-contaminate cooking utensils.

Giardia is one of the most common causes of waterborne and foodborne illness in the United States. It’s usually found in water or food that’s been contaminated with feces containing the parasite. An infection with Giardia is known as giardiasis.

When it comes to food, you get giardiasis most commonly by eating undercooked pork, lamb, or wild game.

Symptoms of infection include cramps, gas, diarrhea, and nausea. It can take as many as 1 to 2 weeks for symptoms to appear and 2 to 6 weeks for them to subside. In rare cases, symptoms can last months or even years.

How to prevent it

You can prevent giardiasis by:

  • washing your hands frequently
  • drinking water from treated municipal sources
  • not swallowing water when swimming
  • thoroughly cooking your meat

There are several kinds of tapeworms that can find their way into the body through food.

Most of the tapeworms that affect humans come from eating undercooked animal products — particularly beef and pork — as well as contaminated fish that’s raw or undercooked.

Symptoms can be nonexistent: People can live with a tapeworm and not know for months or even years.

When you have a tapeworm infection, you may experience weight loss, abdominal pain, and irritation of the anus.

How to prevent it

You can prevent tapeworm infection by thoroughly cooking all the meat you eat, and by washing all fruits and vegetables before you eat or cook with them.

An existing infection of pork tapeworm can be made worse by itching and poor hygiene — where eggs are transferred from the anus to the mouth after scratching or wiping.

There’s a reason why you were taught to always wash your hands after handling animals. Toxoplasma gondii, a microscopic parasite that causes the disease toxoplasmosis, can only reproduce inside of cats. It reaches the rest of the world through cat feces.

If you touch a cat with an infection or handle its litter box without washing your hands afterward, you could easily transmit the parasite to your food when you handle or prepare it.

Symptoms are flu-like, and the U.S. Department of Agriculture (USDA) reports that toxoplasmosis is the third leading cause of death by foodborne illness in the world.

You might also get this parasitic infection from eating undercooked meat or drinking untreated water.

How to prevent it

The best ways to prevent infection are to thoroughly wash and cook your food, wash your hands frequently, and wear gloves when handling cat feces.

Ascaris, a genus of intestinal roundworm, are generally transmitted when people ingest the eggs of the worm. These eggs can end up in your food when you touch contaminated soil or eat fruits and vegetables that were grown in such soil without washing them first.

Symptoms of ascariasis are often mild or nonexistent but may include nausea, vomiting, and abdominal pain, as well as coughing and shortness of breath.

How to prevent it

You can prevent an infection by washing your hands frequently, washing all produce before you eat it, and avoiding any produce you suspect may have been grown in contaminated soil.

Protected by a hard shell, Cryptosporidium parasites are found in fresh produce, milk, and fruit juice.

Cryptosporidiosis (Crypto), an infection with the parasite, can cause an upset stomach, low fever, cramps, and watery diarrhea. These symptoms usually appear 2 to 10 days (or an average of 7 days) after ingestion.

If you come into contact with feces containing the parasite (when changing a diaper, for instance), you could contract an infection.

How to prevent it

You can prevent Crypto by thoroughly washing all of your produce, drinking pasteurized milk and juices, and washing your hands frequently throughout the day.

There are several varieties of flukes or flatworms that may be found in fish, such as Opisthorchiidae and Paragonimus. These worms are killed during the cooking process, so your greatest chance of ingesting one is by eating raw fish.

Symptoms vary depending on the species. They may take months to show up, but they most often include digestive distress.

How to prevent it

While you might be tempted to swear off the sashimi, the chances of infection are fortunately quite low. This is especially true in pricier “sushi-grade” seafood.

When traveling out of the country, you’re cautioned against eating raw freshwater fish and dishes where the preparation methods are unknown.

Pinworms, which are also known as threadworms, include species such as Vibrio vulnificus, Shigella, and Trichinella. They cause the most common worm infection in the United States.

While pinworms usually only affect children, anyone is at risk of infection. Pinworms may make their way into food due to poor hygiene — a child not washing their hands, for example. They are also very easily spread. If one member of a household contracts the infection, then everyone in the household has to be treated.

The worms live for about 5 to 6 weeks in the intestines before dying, leaving behind eggs that hatch and take up residence.

The most common symptom of a pinworm infection is itching around the anus — a symptom that can lead to greater infestation as children scratch the affected area and transport worms and eggs back up to the mouth and face.

How to prevent it

Though mostly harmless, pinworms are generally treated with medication and can be avoided by using improved hygiene practices.


Are there any bacteria or parasites that are beneficial to eat?

A Healthline reader


Yes, there are beneficial bacteria and parasites. Probiotics are beneficial bacteria for humans. Probiotics are live microorganisms found in fermented foods and drinks, such as kombucha, kimchi, sourdough, and sauerkraut. They have been found to help improve digestive health, heart health, immune function, skin health, weight management, and mental health.

There has also been some research showing that parasites may be more beneficial to humans than once thought. Scientists have been looking at how parasites may be helpful in the treatment of immune-mediated diseases or gastrointestinal diseases, such as ulcerative colitis.

Adrienne Seitz, MS, RD, LDNAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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