What is the main cause of bed bugs?

The main cause of bed bugs is introduction through hitchhiking on luggage, clothing, used furniture, and other items brought into home, not poor hygiene, because bed bugs are not caused by dirt but are brought in from infested places. The primary ways bed bugs are introduced include travel, staying in infested hotels, hostels, and rentals where bugs hide in mattress seams and headboards and crawl into luggage and clothing, then are brought home, which is the most common cause, especially after summer travel. Bringing home used mattresses, bed frames, sofas, and other second hand furniture that is infested is a major cause, as bugs hide in cracks. Visiting infested homes and having guests who have bed bugs can spread them via bags and clothing. In apartment buildings, bed bugs travel through shared walls, electrical outlets, and hallways from neighboring infested units. Laundry facilities and shared laundry rooms can spread them via clothing. Clutter provides more hiding places but does not cause them. Bed bugs are attracted to warmth, carbon dioxide, and blood, not dirt, so even spotless five star hotels can have them. Once introduced, a single pregnant female can lay 200 to 500 eggs and infestation grows quickly to hundreds in months. So main cause is human movement and introduction via belongings, not hygiene, which is why inspection of second hand furniture and washing clothes on high heat after travel are key prevention measures.