Infestations fall into three broad categories: those that affect the human body, those that damage homes and buildings, and those that destroy crops and agricultural land. Each category includes dozens of specific pests, but understanding the major types helps you recognize what you’re dealing with and how serious the problem is.
Infestations That Affect the Human Body
Human infestations involve parasites that live either on or inside the body. The ones that live on the surface, called ectoparasites, are the most immediately noticeable. The ones that live inside, primarily worms, can go undetected for much longer.
Lice
Head lice spread through direct head-to-head contact or by sharing hats, scarves, and headbands. In an active infestation, both adult lice and their eggs (called nits) are present. Adults are brown and roughly the size of a sesame seed, while eggs are clear and cling to individual hair shafts. Lice don’t transmit diseases directly, but heavy infestations cause intense itching and can lead to secondary skin infections from scratching. Body lice, a separate species, can carry typhus and relapsing fever.
Scabies
Scabies is caused by a microscopic mite that burrows into the top layer of skin, triggering severe itching that typically worsens at night. It spreads through prolonged skin-to-skin contact or through infested bedding and towels. A doctor or dermatologist can confirm the diagnosis, sometimes by taking a small skin scraping to identify the mite under a microscope. Scabies affects people across all income levels and living situations.
Bed Bugs
Bed bugs feed on human blood at night and retreat into mattress seams, headboards, and furniture crevices during the day. Their bites often appear in a zigzag pattern on the skin. Other signs include flecks of blood on sheets, tiny white eggs, dark spots of excrement on bedding, and a sweet, musty odor in the room. Bed bugs don’t transmit disease, but infestations cause sleep disruption, anxiety, and allergic reactions in some people.
Internal Parasitic Worms
Parasitic worms that live inside the body fall into three groups: roundworms, tapeworms, and flukes. Most live in the digestive tract, where they feed on nutrients or blood. Some, however, migrate to other organs. Certain roundworm species settle in the lymphatic system, under the skin, or even in the eyes. Adult worms range from just over a millimeter to more than a meter long. These infestations are more common in tropical regions but occur worldwide, often spreading through contaminated food, water, or soil.
Disease-Carrying Pest Infestations
Some of the most dangerous infestations involve insects that carry pathogens between animals and humans. Vector-borne diseases account for more than 17% of all infectious diseases globally, causing over 700,000 deaths each year.
Mosquitoes are the deadliest vector. Different species transmit malaria (an estimated 249 million cases and over 608,000 deaths annually), dengue (roughly 96 million symptomatic cases per year), Zika, yellow fever, West Nile virus, and several other diseases. Ticks carry Lyme disease, several types of spotted fever, and tick-borne encephalitis. Fleas transmit plague bacteria from rodents to humans. Even sandflies carry parasites that cause leishmaniasis, a disease affecting the skin or internal organs.
What makes these infestations particularly concerning is that a single bite can be enough. You don’t need a heavy infestation in your home to be at risk if you live in or travel to an area where these vectors are active.
Household and Structural Infestations
Household infestations damage property, contaminate food, and create health hazards. They generally fall into three groups: insects, rodents, and wood-destroying organisms.
Rodents
Mice and rats are the most common structural pests. They gnaw through wiring (creating fire hazards), contaminate food stores, and leave droppings that can trigger allergic reactions and spread disease. Signs of a rodent infestation include scratching or scurrying noises in walls and ceilings, small burrows near your home’s foundation, grease marks along baseboards, gnawed packaging, and a musty odor from nesting materials.
Cockroaches
Cockroach infestations are identified by droppings, shed skins found behind appliances or under sinks, and a distinctive oily smell. They contaminate food surfaces and are a well-established trigger for asthma and allergies, especially in children. Cockroaches reproduce rapidly, so a small problem can become a large one within weeks.
Wood-Destroying Insects
Termites are the most economically significant structural pest in the United States. U.S. residents spend an estimated $5 billion annually on termite control and repairs, with the average homeowner paying around $3,000 to fix termite damage once it’s discovered. When combined with damage to crops and other structures, termites and similar pests cause roughly $30 billion in losses per year.
Subterranean termites, the most common type, require constant contact with soil moisture. They build mud tubes to bridge the gap between the ground and the wood they feed on, so pencil-width mud tunnels along your foundation are a key warning sign. Carpenter ants are another wood-destroying pest, but unlike termites, they don’t eat wood. They excavate it to build nests, and their presence usually signals a moisture problem in the structure. Carpenter bees bore round holes into unpainted exterior wood for nesting and will reuse the same site across generations, gradually worsening the damage.
Wood-destroying beetles can enter a home through untreated building materials, secondhand furniture, or firewood brought indoors. Some species, like wharf borers, target buried or moisture-damaged wood specifically.
Agricultural and Crop Infestations
Crop infestations threaten food supplies and cost billions in lost yield and pest control. The U.S. Department of Agriculture classifies plant pests into four main categories: insects and mites, nematodes (microscopic worms that attack roots), mollusks like slugs and snails, and noxious weeds that outcompete crops for water and nutrients.
Insect pests are the most diverse group. Aphids, caterpillars, beetles, and locusts all damage crops differently, from sucking sap out of leaves to boring into fruit to stripping entire fields bare. Nematodes are less visible but equally destructive. They live in soil and attack root systems, stunting growth and making plants vulnerable to secondary infections. Invasive weeds, while not parasites in the traditional sense, are classified as a type of infestation because they spread aggressively and are difficult to eradicate once established.
Plant diseases caused by fungi, bacteria, and viruses often accompany pest infestations. Insects that feed on crops can introduce pathogens as they move from plant to plant, turning a moderate pest problem into a disease outbreak. Citrus diseases, potato blight, stem rust in wheat, and dozens of other plant diseases are tracked and managed at the federal level precisely because they can devastate entire growing regions.
How to Identify an Infestation Early
Regardless of the type, most infestations share a few common warning signs. Physical damage is the most obvious: chewed wiring, gnawed furniture, unexplained holes in walls or floors, or damaged plants. Droppings, shed skins, and grease marks in hidden areas like crawl spaces or behind appliances point to insects or rodents. Unusual sounds, particularly scratching or rustling inside walls, suggest rodents or squirrels. A persistent musty or foul odor that you can’t trace to another source often comes from pest nests or accumulated droppings.
On your body, unexplained itching, bites in patterns (especially zigzag lines), or skin irritation that doesn’t respond to typical treatments may indicate bed bugs, scabies, or another ectoparasite. Household members experiencing new allergic reactions or respiratory symptoms without an obvious cause should also consider dust mites or cockroach allergens as a possibility.
Managing and Eliminating Infestations
The most effective approach to any infestation is integrated pest management, or IPM, a strategy built around six core steps: identifying the pest correctly, monitoring its numbers and damage, determining whether the problem has crossed a threshold that requires action, preventing conditions that support the pest, combining multiple control methods, and evaluating the results after treatment.
Prevention is the foundation. For household pests, that means sealing entry points, reducing moisture, storing food in airtight containers, and eliminating clutter where pests hide. For agricultural pests, it includes rotating crops, choosing pest-resistant plant varieties, and adjusting irrigation to avoid conditions that favor disease.
When active control is needed, IPM uses biological, physical, and chemical tools in combination. Biological control introduces natural predators or pathogens that target the pest. Physical controls include traps for rodents, screens to block insects, and mulch to suppress weeds. Chemical pesticides are the last resort, chosen and applied to minimize harm to people, non-target species, and the surrounding environment. This layered approach works for every type of infestation, from a cockroach problem in an apartment to a nematode outbreak in a soybean field.