Bed bugs and their relatives occur nearly worldwide. Bed bugs became relatively scarce during the latter part of the 20th century, but their populations have resurged in recent years, particularly throughout parts of North America, Europe, and Australia. They are most abundant in rooms where people sleep, and they generally hide nearest the bed or other furniture used for sleeping. Bed bugs are most active in the middle of the night, but when hungry, they will venture out during the day to seek a host. Their flattened bodies allow them to conceal themselves in cracks and crevices around the room and within furniture. Favored hiding sites include the bed frame, the seams around mattresses and undersides of box springs. Clutter around the room offers additional sites for these bugs to hide, and increases the difficulty in eliminating bed bugs once they have become established. Bed bugs also have been found in schools, health care facilities, trains and buses, offices and yet other kinds of structures.