Bed bug infestations are primarily the result of two species of insects from genus Cimex: Cimex lectularius (the common bed bug) and Cimex ] These insects feed exclusively on blood and may survive a year without eating. Adult Cimex are light brown to reddish-brown, flat, oval, and have no hind wings. The front wings are vestigial and reduced to pad-like structures. Adults grow to 4–5 mm (0.16–0.20 in) long and 1.5–3 mm (0.059–0.118 in) wide.
Bed bugs have five immature nymph life stages and a final sexually mature adult stage. They shed their skins through ecdysis at each stage, discarding their outer exoskeleton. Newly hatched nymphs are translucent, lighter in color, and become browner as they moult and reach maturity. Bed bugs may be mistaken for other insects, such as booklice, small cockroaches, or carpet beetles; however, when warm and active, their movements are more ant-like, and like most other true bugs, they emit a characteristic disagreeable odor when crushed.
Bed bugs are obligatory bloodsuckers. They have mouth parts that saw through the skin, and inject saliva with anticoagulants and painkillers. Sensitivity of humans varies from extreme allergic reaction to no reaction at all (about 20%). The bite usually produces a swelling with no red spot, but when many bugs feed on a small area, reddish spots may appear after the swelling subsides. Bedbugs prefer exposed skin, preferably the face, neck, and arms of a sleeping person.
Bed bugs are attracted to their hosts primarily by carbon dioxide, secondarily by warmth, and also by certain chemicals. Cimex lectularius only feeds every five to seven days, which suggests that it does not spend the majority of its life searching for a host. When a bed bug is starved, it leaves its shelter and searches for a host. It returns to its shelter after successful feeding or if it encounters exposure to light. Cimex lectularius aggregate under all life stages and mating conditions. Bed bugs may choose to aggregate because of predation, resistance to desiccation, and more opportunities to find a mate. Airborne pheromones are responsible for aggregations.
Infestation is rarely caused by a lack of hygiene. Transfer to new places is usually in the personal items of the human they feed upon. Dwellings can become infested with bed bugs in a variety of ways, such as:
Bugs and eggs inadvertently brought in from other infested dwellings on a visiting person's clothing or luggage;
Infested items (such as furniture especially beds or couches, clothing, or backpacks) brought in a home or business;
Proximity of infested dwellings or items, if easy routes are available for travel, e.g. through ducts or false ceilings;
Wild animals (such as bats or birds) that may also harbour bed bugs or related species such as the bat bug;
People visiting an infested area (e.g. dwelling, means of transport, entertainment venue, or lodging) and carrying the bugs to another area on their clothing, luggage, or bodies. Bedbugs are increasingly found in air travel.
Though bed bugs will opportunistically feed on pets, they do not live or travel on the skin of their hosts, and pets are not believed to be a factor in their spread
Bed bugs can exist singly, but tend to congregate once established. Although strictly parasitic, they spend only a tiny fraction of their life cycles physically attached to hosts. Once a bed bug finishes feeding, it relocates to a place close to a known host, commonly in or near beds or couches in clusters of adults, juveniles, and eggs—which entomologists call harborage areas or simply harborages to which the insect returns after future feedings by following chemical trails. These places can vary greatly in format, including luggage, inside of vehicles, within furniture, among bedside clutter—even inside electrical sockets and nearby laptop computers. Bed bugs may also nest near animals that have nested within a dwelling, such as bats, birds, or rodents. They are also capable of surviving on domestic cats and dogs, though humans are the preferred host.