Parasites are organisms that live by stealing nutrients from their hosts, and they often sound like something straight out of a sci-fi movie (picture that chest-bursting scene from Alien).

But the reality is, parasitic infections are a lot more common in humans than you think. In fact, malaria is caused by a parasite, and that disease kills 600,000 people every year.

The types of parasite you’re probably most familiar are called helminths and protozoa, and they cause gastrointestinal symptoms, according to Pascale M. White, M.D., an assistant professor of medicine and director of the Gastroenterology Clinic in the Dr. Henry D. Janowitz division of gastroenterology at the Icahn School of Medicine at Mount Sinai. You’ve definitely heard of helminth infections like tapeworms, roundworms, hookworms, and pinworms, and you might even heard of Giardia, a protozoa parasite that causes diarrheal disease. These parasites are the kind of thing you might joke about if you’re having serious stomach issues—”maybe I picked up a tapeworm, lol”—but they’re not really a joking matter.

While some parasitic infections are easily cured, others can be chronic, debilitating, and even fatal. More than 300,000 people are living with Trypanosoma cruzi, the parasitic cause of Chagas disease (a life-threatening illness that causes a wide variety of symptoms), according to the Centers for Disease Control and Prevention; at least 1,000 people are hospitalized with neurocysticercosis, a type of tapeworm, each year; more than 60 million people are chronically infected with Toxoplasma gondii, a parasitic disease that can cause flu-like symptoms; and 1.1 million people are newly infected with Trichomonas, a parasitic STD, each year. Not so funny anymore, huh?

If you thought parasites were the kind of thing you could only pick up while traveling abroad, think again. “Humans can get infected by ingesting the parasite or its larvae (eggs) found in fecal contaminated food or water,” says White. “Another way is by ingesting the parasite larvae found in undercooked meats like pork or raw fish. And, sometimes, certain parasites have the ability to directly penetrate bare human skin when a person comes into contact with fecal contaminated soil or water.” All of those things are just as likely to happen at home in the United States as they are while you’re beach-hopping in Thailand. Plus, there are certain types of parasites that are more common in America. “For example, Necatur americanus is a hookworm species that is found in the U.S.,” says White. “Strongyloides is a roundworm that is found in hot humid climates in southeastern parts of the United States.”

So how do you know if you have a parasite versus some nasty stomach bug? To be honest, it's tough to tell. There are over 1,000 types of parasites out there, and the symptoms can vary—but will most likely seem unrelated and random. But most parasitic infections do share a number of similar symptoms, including diarrhea, abdominal cramping or pain, nausea, vomiting, lack of appetite, generalized fatigue, unexplained rashes or skin irritation, cough, and unintentional weight loss.

If that list seems…broad, well, you’re not wrong. The symptoms of parasitic infections have a lot in common with symptoms for pretty much any other ailment. The key in determining a parasite source, White says, is the combination of symptoms and the duration of the symptoms. “Keep in mind that some parasites can live in the human body for months or years without causing any symptoms at all!” she adds. “So getting a good medical history and travel history is key. We take all of these signs and symptoms into account when we consider the diagnosis of a parasitic infection.”

If you’ve noticed any of these symptoms and you’ve recently traveled abroad, come into contact with unclean animals, eaten something that could have been contaminated, or genuinely can’t come up with any other reason why you’d be experiencing that reaction, get yourself to primary care doctor or a gastroenterologist ASAP. They can do stool and blood tests to determine what’s causing your symptoms. “For example, certain parasitic infections can lead to abnormalities in specific blood tests, causing conditions like anemia, eosinophilia, or elevated liver enzymes,” explains White. Once they uncover the root issue, a doctor or gastroenterologist can prescribe the right meds to deal with it.

While a real-life parasitic infection may not lead to an Alien-like climax, some of them can be downright unpleasant to deal with (did you see that list of symptoms?). The best thing you can do is identify it and treat it—fast, so you can move on with your life.