Bedbugs lurk in cracks and crevices and have been living on human blood for centuries. Though they aren’t known to transmit disease or pose any serious medical risk, the stubborn parasites can leave itchy and unsightly bites, but bedbugs don’t always leave marks. The best way to tell if you have a bedbug infestation is to see the live, apple-seed-size critters for yourself. Unfortunately, once bedbugs take up residence in homes and businesses, they can be difficult to exterminate without professional help.

Bedbugs are flat, round and reddish brown. The ones that typically plague humans are the common bedbug (Cimex lectularius) and the tropical bedbug (Cimex hemipterus).

The creatures don’t have wings and they can’t fly or jump, but their narrow body shape and ability to live for months without food make them ready stowaways and squatters. Bedbugs can easily hide in the seams and folds of luggage, bags and clothes. They also take shelter behind wallpaper and inside bedding, box springs and furniture. The ones that feed on people can crawl more than 100 feet (30 meters) in a night, but typically creep to within eight feet (2.4 m) of the spot its human hosts sleep, according to the CDC.

Bedbugs reproduce by a gruesome strategy appropriately named “traumatic insemination,” in which the male stabs the female’s abdomen and injects sperm into the wound. During their life cycle, females can lay more than 200 eggs, which hatch and go through five immature “nymph” stages before reaching their adult form, molting after each phase.

Bedbugs feed on the blood of humans (though some species have a taste for other mammals and birds, too) by inserting a sharp proboscis, or beak, into the victim’s skin. The critters engorge with blood in about 10 minutes, which fills them up for days.

The insects are most active at night, though they are not exclusively nocturnal. Bedbugs are attracted to warmth, moisture and the carbon dioxide released from warm-blooded animals, according to Purdue University. On sleeping human hosts, bedbugs often bite exposed areas of the body, such as the face, neck, arms and hands.

But looking for bedbug bites might not be the best way to tell if you have an infestation.

“A lot of people put a lot of import on looking at the bite and identifying it,” Harold Harlan, an entomologist and a bedbug expert, told Live Science. “I’ve raised these things for 41 years and I cannot tell what is a bedbug bite.”

Bedbug bites can look very similar to bites from other insects like mosquitoes and fleas. People also have widely varying reactions to bedbug bites. Some people have little visible reaction to the insects’ nibbling — they don’t develop lesions or bumps or pustules at all.

The bites themselves don’t usually pose any major health risk since bedbugs are not known to spread diseases, but an allergic reaction to the bites may require medical attention, CDC officials say. There have also been some strange cases linked to bedbug infestations. Researchers reported in the Canadian Medical Association Journal in 2009 that they treated a 60-year-old man for anemia caused by blood loss from bedbug bites. Another study published in 1991 in the Journal of the Egyptian Society of Parasitology found that people with asthma might be more susceptible to allergic reactions from bedbug bites.

If bites are unreliable markers of an infestation, how can you tell if you have bedbugs? Seeing live, moving bugs is the “gold standard,” according to Harlan. If you can, you should collect some of those specimens in a closed container and get a professional to identify them.

You should look for traces of the insects in the folds of your mattresses, box springs and other places where they are likely to hide. You might be able to find their papery skins, which get cast off after molting, which look like popcorn kernels but are smaller and thinner, Harlan said. They also leave small, dark-colored spots from the blood-filled droppings they deposit on mattresses and furniture. If you can touch the spot with a water-soaked towel and it runs a rusty, reddish color, you’re probably looking at a fresh drop of bedbug feces, Harlan said.

Bedbugs often invade new areas by hitching a ride on clothing, luggage, furniture or bedding. The creatures don’t discriminate between dirty and clean homes, which means even luxury hotels can be susceptible to bedbugs. The most at-risk places tend to be crowded lodgings with high occupant turnover, such as dormitories, apartment complexes, hotels and homeless shelters.

Getting rid of clutter may help to reduce the number of hiding places for bedbugs, but according to the CDC, the best way to prevent bedbugs is regular inspection for the signs of an infestation.