Before you self-diagnose your symptoms as due to a parasite, it’s important to understand what a parasite is. “A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host,” according to the CDC. There are three classes of parasites: Protozoa, which are tiny, one-celled organisms that typically live in the intestines, blood, or tissue; helminths, which are parasitic worms such as tapeworms, roundworms, and thorny-headed worms; and ectoparasites, which are ticks, fleas, lice, and mites that attach to or burrow into the skin.

Parasites can cause disease and even death, but fortunately, if caught early, the infections can usually be treated with medication. (And sometimes they are just annoying and icky, but not necessarily life-threatening—like head lice.) Here are some silent signs and parasite symptoms that could mean you are hosting one of these tiny invaders, including the parasite that millions of people in the United States have and don’t know it.

You ingested (or swam in) some questionable water

Whether you were traveling abroad and drank from the tap or went to the lake around the block and splashed around for a bit, ingesting contaminated water is one of the most common causes of parasites, according to Daliah Wachs, MD, a board-certified family physician in Las Vegas. In fact, before you drink that next glass of water, here are some things you need to know about whether or not your tap water is safe.

You enjoy a rare steak

Tend to prefer your foods on the rare side? It might not be the best option if you’re looking to avoid a parasite. “Ingesting raw or undercooked beef or pork can lead to [infection by] the Taenia type of intestinal tapeworms,” says Dana Hawkinson, MD, an assistant professor at the University of Kansas School of Medicine. “Eating certain raw fish can lead to [infection by] diphyllobothrium, [a type] of intestinal tapeworm. Additionally, ingestion of contaminated food or water can lead to an Ascaris infection.” Ascaris intestinal roundworms can grow up to 35 centimeters in length and lead to intestinal blockage, so you definitely want to avoid them.

You’re losing weight

There are many diseases that cause unexplained weight loss, but losing weight is also one of the primary parasite symptoms for certain types of these freeloaders. “Tapeworms will cause you to lose weight because you have this huge worm in your intestines eating your food,” Patricia Quinlisk, MD, MPH, former Iowa Department of Public Health Medical Director/State Epidemiologist, wrote in an email newsletter she sends to healthcare providers. The weight loss is often accompanied by loss of appetite and upset stomach. Still, there are other health reasons to explain why you could be rapidly losing weight.

You have a compromised immune system

A compromised immune system could inhibit your body’s ability to fight off a parasite. “Those with diabetes and HIV, as well as those undergoing transplants who are on immunosuppressants, are most at risk,” Dr. Wachs says.

You’re spending a lot of time in the bathroom

This is also one of the more common parasite symptoms noticed by hosts. According to the CDC, Giardia infection is one of the most common intestinal parasitic disease affecting humans in the United States—you might get it from drinking Giardia-infested water, eating raw food that contains the parasite, touching surfaces contaminated with Giardia (like bathroom handles or diaper pails), or swimming in Giardia-infested lakes, rivers, or streams. It can also be transmitted through the fecal-oral route via inadequate hand hygiene, daycare centers, and oral-anal sex. If you’ve got it, you’ll be quick to notice that something is amiss. The infection is known for causing watery, smelly and fatty diarrhea, gas, abdominal cramps, and dehydration within one to three weeks after exposure.